Sirens sounded in an Israeli kibbutz after mortar shells were fired from the Gaza Strip on Tuesday morning.

JERUSALEM — Gaza militants fired at least 27 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.

Israel’s Iron Dome antimissile system intercepted most of the projectiles, according to the military, and there were no casualties, but this was the heaviest barrage fired out of the Palestinian coastal territory since the 50-day war in the summer of 2014. Israel has responded to events along the border in recent weeks 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…