JERUSALEM — Gaza militants fired at least 28 mortar shells into southern Israel on Tuesday morning, setting off sirens, activating Israel’s air defenses and sharply raising the stakes after weeks of deadly protests, arson attacks and clashes along the border.
The barrage was the heaviest to be fired out of the Palestinian coastal territory since the 50-day war in the summer of 2014, although Iron Dome antimissile system intercepted most of the projectiles on Tuesday, according to the military, and there were no casualties.
Israel responded later in the day with airstrikes on targets in Gaza, after Prime Minister Benjamin Netanyahu said that Israel held Hamas responsible for the mortar attacks and that the military would respond “forcefully.”
Tensions have been spiraling along the border in recent weeks, during a Palestinian protest campaign called the Great Return March. The protests were intended to challenge the 11-year blockade of the Gaza Strip and to press Palestinian claims to lands in what is now Israel, which has responded to certain events with airstrikes against military targets in Gaza.
One of the mortar shells fired around 7 a.m. crashed into the yard of a kindergarten in an Israeli border community shortly before the children were to arrive. Television images showed the fortified walls of the kindergarten pockmarked with shrapnel; hunks of metal from the mortar shell jutted out of the sand in the playground.
Soon after the initial barrage of 25 mortars, the Israeli authorities announced a return to normal and schools and kindergartens in the area opened, suggesting that the military was not expecting, or planning, an immediate escalation into a broader conflict.
Half an hour later, sirens sounded again as two more mortars were launched. Around 9:30 a.m., sirens blared again, and a new mortar landed in open ground.
There was no immediate claim of responsibility from Gaza militant groups, but suspicion immediately fell on Islamic Jihad, an extremist group backed by Iran that sometimes rivals Hamas, the Islamic militant group that controls the territory, but sometimes works in concert with it against Israel.
“Israel views harshly the attacks against it and its communities by Hamas and Islamic Jihad from Gaza,” Mr. Netanyahu said. “Israel will exact a heavy price from whoever tries to harm it, and we view Hamas as bearing the responsibility for preventing such attacks against us.”
Avigdor Lieberman, the Israeli defense minister, said he would meet in Tel Aviv on Tuesday with the military chief of staff and other security chiefs to assess the situation.
The United Nations special coordinator for the Middle East peace process, Nickolay E. Mladenov, condemned the barrage of fire out of Gaza. “Such attacks are unacceptable and undermine the serious efforts by the international community to improve the situation in Gaza,” he said in a statement. “All parties must exercise restraint, avoid escalation and prevent incidents that jeopardize the lives of Palestinians and Israelis.”
The European Union ambassador to Israel, Emanuele Giaufret, wrote on Twitter, “I know the resilience of communities in southern Israel but indiscriminate attacks are totally unacceptable and to be condemned unreservedly.”
On Sunday, the Israeli military shelled an Islamic Jihad observation post in southern Gaza, killing three members of the group, in what the military said was a response to an explosive being planted the night before along the security fence dividing Gaza from Israel. The bomb, hidden in a pair of wire cutters, exploded as sappers neutralized it remotely. There were no injuries on the Israeli side.
Islamic Jihad vowed to respond. Israeli commentators said on Tuesday that the group was unlikely to have been able to respond on the scale of that morning’s barrage without the approval of Hamas.
As many as 120 Palestinians have been killed by Israeli fire since March 30, according to the Gaza Health Ministry, mostly by snipers during the protests and half of them in a single day, May 14, the peak of the campaign.
That was timed to coincide with the contentious opening of the United States Embassy in Jerusalem and came on the eve of the 70th anniversary of what Palestinians call the nakba, or catastrophe, referring to the war surrounding Israel’s establishment in 1948.
Israel said it was defending its border and the nearby communities against a mass breach by the protesters, adding that Gaza militants intended to use unarmed civilian protesters as cover to infiltrate Israeli territory and attack Israeli soldiers and civilians.
In addition, Palestinians have sent incendiary kites and balloons over the border fence from Gaza, setting hundreds of fires in the fields and forest on the Israeli side.
Over the weekend, a small group of Palestinians cut through the fence and set fire to an empty Israeli military post. Israeli fighter jets struck a Hamas military compound in response. And late Monday, heavy machine-gun fire from Gaza hit buildings in the Israeli border town of Sderot.
Iyad Abuheweila contributed reporting from Gaza.