It was not immediately clear if the attack was the prelude to a ground invasion against Hamas, the Islamist militant group that controls Gaza.
An Israeli military spokesman, Lt. Col. Jonathan Conricus, initially said that “there are ground troops attacking in Gaza,” but later clarified that Israeli troops had not entered Gaza, suggesting the possibility of artillery fire from the outside. He provided no further details.
The surge in fighting highlighted the unprecedented position Israel finds itself in — battling Palestinian militants on its southern flank as it seeks to head off its worst civil unrest in decades.
It followed another day of clashes between Arab and Jewish mobs on the streets of Israeli cities, with the authorities calling up the army reserves and sending reinforcements of armed border police to the central city of Lod to try to head off what Israeli leaders have warned could become a civil war.
Taken together, the two theaters of turmoil pointed to a step change in the grinding, decades-old conflict between Israel and the Palestinians. While violent escalations often follow a predictable trajectory, this latest bout, the worst in seven years, is rapidly evolving into a new kind of war — faster, more destructive and capable of spinning in unpredictable new directions.
In Gaza, an impoverished coastal strip that was the crucible of a devastating seven-week war in 2014, Palestinian militants fired surprisingly large barrages of enhanced-range rockets — some 1,800 in three days — that reached far into Israel.
Israel intensified its campaign of relentless airstrikes against Hamas targets there on Thursday, pulverizing buildings, offices and homes in strikes that have killed 103 people including 27 children, according to the Gaza health authorities.
Six civilians and a soldier have been killed by Hamas rockets inside Israel.
Egyptian mediators arrived in Israel Thursday in a sooner-than-usual push to halt the spiraling conflict.
Most alarming for Israel, though, was the violent ferment on its own sidewalks and streets, where days of rioting by Jewish vigilantes and Arab mobs showed no sign of abating.
The unrest in several mixed-ethnicity cities, where angry young men stoned cars, set fire to mosques and synagogues, and attacked each other, signaled a collapse of law and order inside Israel on a scale not seen since the start of the second Palestinian uprising, or intifada, 21 years ago.
The violence follows a month of boiling tensions in Jerusalem, where the threatened eviction of Palestinian families from their homes coincided with a spate of Arab attacks against Israeli Jews, and a march through the city by right-wing extremists chanting “Death to Arabs.”
The jarring violence this week caused Israeli leaders, led by President Reuven Rivlin, to evoke the specter of civil war — a once unthinkable idea. “We need to solve our problems without causing a civil war that can be a danger to our existence,” Mr. Rivlin said. “The silent majority is not saying a thing, because it is utterly stunned.”
Prime Minister Benjamin Netanyahu visited Lod, a working-class city with a mixed Arab-Israeli population that has emerged as the center of the upheaval. Hulks of burned-out cars littered the streets where, a few nights earlier, Arab youths burned synagogues and cars, threw stones and let off sporadic rounds of gunfire, before gangs of Jewish vigilantes counterattacked and set their own fires.
On Thursday, a Jewish man was stabbed as he walked to a synagogue there, but survived.
“There is no greater threat now than these riots,” said Mr. Netanyahu, who vowed to deploy the Israel Defense Forces to keep the peace in Lod. A day earlier, he described the violence as “anarchy” and said: “Nothing justifies the lynching of Jews by Arabs, and nothing justifies the lynching of Arabs by Jews.”
To secure Lod, the government brought in thousands of armed border police from the occupied West Bank, and imposed an 8 p.m. curfew, but to little effect.
Arab residents, who account for about 30 percent of the town’s 80,000 people, continued a campaign of stone-throwing, vandalism and arson, while Jewish extremists arrived from outside Lod, burning Arab cars and property. Arab protesters erected flaming roadblocks.
As night fell there were signs that the violence might escalate when a large convoy of armed Jews in white vans moved into town.
Palestinian leaders, however, said the talk of civil war by Jewish leaders was a distraction from what they called the true cause of the unrest in Lod — police brutality against Palestinian protesters and provocative actions by right-wing Israeli settler groups.
The Israeli-Palestinian Conflict
“The police shot an Arab demonstrator in Lod,” said Ahmad Tibi, the leader of the Ta’al party and a member of Israel’s Parliament. “We don’t want bloodshed. We want to protest.”
Mr. Tibi said that Mr. Netanyahu, who has frequently aligned with far-right and nationalist parties to stay in power, had only himself to blame for the political tinderbox that has exploded with such ferocity across Israel.
On Thursday evening, the State Department urged American citizens to reconsider traveling to Israel and warned against going to the occupied West Bank or Gaza. In an advisory, the department noted rocket attacks that could reach Jerusalem, protests and violence throughout Israel and a “dangerous and volatile” security environment in the Gaza Strip and on its borders.
The trouble started on Monday, when a heavy-handed police raid at Jerusalem’s Al Aqsa Mosque — the third-holiest site in Islam, located atop a site also revered by Jews — set off an instant backlash.
But beyond the images of police officers flinging stun grenades and firing rubber bullets inside the mosque, Palestinian outrage was also fueled by much wider, decades-old frustrations.
Human Rights Watch recently accused Israel of perpetrating a form of apartheid, the racist legal system that once governed South Africa, citing a number of laws and regulations that it said systematically discriminate against Palestinians. Israel vehemently rejected that charge. But its security forces are now confronted with a swelling wave of fury from the country’s Arab Israeli minority, which complains of being treated as second-class citizens.
“‘Coexistence’ means that both sides exist,” said Tamer Nafar, a famous rapper from Lod. “But so far there is only one side — the Jewish side.”
The rocket attacks from Gaza are also quantitatively and qualitatively different from the last war in 2014. The more than 1,800 rockets Hamas and its allies have fired at Israel since Monday already represent a third of the total fired during the seven-week war in 2014.
Israeli intelligence has estimated that Hamas, Islamic Jihad and other Palestinian militant groups have about 30,000 rockets and mortar projectiles stashed in Gaza, indicating that despite the Israeli-Egyptian blockade of the coastal territory, the militants have managed to amass a vast arsenal.
The rockets have also demonstrated a longer range than those fired in previous conflicts, reaching as far as Tel Aviv and Jerusalem.
They have also proven more effective. In the 2014 war, they killed a total of six civilians inside Israel, the same number killed in the last three days.
Those casualties appeared to be a product of Hamas’s new tactic of firing more than 100 missiles simultaneously, thwarting the American-financed Iron Dome missile-defense system, which Israeli officials say is 90 percent effective at intercepting rockets before they land inside Israel.
Gaza residents have no such protection against Israeli airstrikes, which crushed three multistory buildings in the strip after residents were warned to evacuate. Israeli officials said that the buildings housed Hamas operations and that they were striving to limit civilian casualties, but many Gaza residents viewed the Israeli attacks as a form of collective punishment.
Thursday was supposed to be a day of celebration for Palestinians as they marked the end of the holy month of Ramadan, a day when Muslims typically gather to pray, wear new clothes and share a family meal. In Jerusalem, tens of thousands of worshipers gathered at dawn outside the Aqsa Mosque, some waving Palestinian flags and a banner showing an image of Ismail Haniyeh, the leader of Hamas.
In Gaza, though, it was a somber day of funerals, fear and missile strikes. Some families buried their dead, others laid out prayer mats beside buildings recently destroyed in Israeli airstrikes, and still others came under attack from Israeli drones hovering overhead.
“Save me,” pleaded Maysoun al-Hatu, 58, after she was wounded in a missile strike outside her daughter’s house in Gaza, according to a witness. An ambulance arrived moments later, but it was too late. Ms. al-Hatu was dead.
American and Egyptian diplomats were heading to Israel to begin de-escalation talks. Egyptian mediators played a key role in ending the 2014 war in Gaza, but this time there is little optimism they can achieve a quick result.
Israeli military officials have said their mission is to stop the rockets from Gaza, and the military moved tanks and troops into place along the border with Gaza on Thursday in preparation for a possible ground invasion.
The decision to extend the campaign is ultimately political. Analysts said that a ground operation would likely incur high casualties, and it was unclear if the troop deployment was anything more than a threat.
But the political calculation grew more complicated on Thursday after the collapse of negotiations between opposition parties seeking to form a new government.
Naftali Bennett, an ultranationalist former settler leader who opposes Palestinian statehood, pulled out of the talks, citing the state of emergency in several Israeli cities.
His withdrawal increases the likelihood of Israel holding a general election later this summer — in what would be its fifth in just over two years. And the collapse of the talks appears to benefit Mr. Netanyahu, making it impossible for opposition parties to form an alliance large enough to oust him from office.
Mr. Netanyahu, who is on trial on corruption charges, is serving as caretaker prime minister until a new government can be formed.
On the Palestinian side, the indefinite postponement last month of elections by the Palestinian president, Mahmoud Abbas, created a vacuum that Hamas is more than willing to fill.
Isabel Kershner contributed reporting from Lod, Israel; Iyad Abuheweila from Gaza City; Patrick Kingsley, Irit Pazner Garshowitz and Myra Noveck from Jerusalem; Gabby Sobelman from Rehovot, Israel; Mona el-Naggar and Vivian Yee from Cairo; Megan Specia from London; Steven Erlanger from Brussels; and Lara Jakes from Washington.