JERUSALEM — Islamic militants in Gaza attacked southern Israel with rockets and mortars on Tuesday and Israel responded instantly with a wave of airstrikes across the Palestinian territory, a sharp escalation of violence after weeks of deadly protests, arson attacks and armed clashes along the border.

The exchanges were the most intense cross-border hostilities in Gaza since the two sides fought a 50-day war in the summer of 2014. By 10 p.m. on Tuesday, Israel said there had been 70 rockets or mortars fired from Gaza throughout the day.

The military wing of Hamas — the Islamic militant group that rules Gaza — claimed responsibility jointly with another faction, the Palestinian Islamic Jihad. Earlier in the day, it was initially assumed that Islamic Jihad was behind most of the attacks after Israel killed three of its members on Sunday.

“Bombing will be met by bombing and blood for blood,” the two groups said in the joint claim.

Israel responded with airstrikes on 35 targets in Gaza after Prime Minister Benjamin Netanyahu said that Israel held Hamas responsible for the mortar attacks. Israel’s Iron Dome antimissile system intercepted dozens of incoming projectiles, according to the military.