An Israeli air strike has killed a senior figure in the militant group Palestinian Islamic Jihad, amid the most serious flare-up in hostilities between Israel and militants in the Gaza Strip for more than a year.

Over the past three days, Israeli forces have repeatedly bombed sites in Gaza — an impoverished stretch of land hemmed in between Israel and Egypt that is home to more than 2mn Palestinians — while Islamic Jihad militants have fired hundreds of rockets into Israel.

According to the health ministry in Gaza, 31 Palestinians, including six children and four women, have been killed since the fighting began. No Israeli deaths have been reported.

Israel said on Sunday that among those killed was Khaled Mansour, who commanded Islamic Jihad’s operations in the south of Gaza and was targeted by an air strike on Saturday. His counterpart in northern Gaza, Taysir al-Jabari, was killed by an Israeli strike on Friday, triggering the latest round of fighting.

The escalation is the sharpest since Israel and Hamas, the militant group which rules Gaza, fought an 11-day war last year.

Israel said it had launched the air raids in response to an “imminent threat” from Islamic Jihad, following days of tension sparked by the arrest of another senior member of the group in the occupied West Bank last week.

Islamic Jihad, an Iran-backed group founded in the 1980s, responded by firing a barrage of rockets into Israel, setting off warning sirens across the south and centre of the country.

Israel’s prime minister, Yair Lapid, said on Sunday that the Israeli military was continuing strikes on “terrorist targets and operatives, and to thwart rocket-launching squads”.

“This operation will continue as long as necessary,” he said.

Gaza’s health ministry said that in addition to the 31 Palestinian deaths, “Israeli aggression” was responsible for injuries to 265 people.

An Israeli official said Israeli strikes had killed 17 militants. Israeli officials also said that one incident, in which children were killed in the Jebaliya refugee camp in northern Gaza, was the result of a rocket launched by Islamic Jihad militants misfiring and falling short of its target.

On Sunday morning Islamic Jihad militants sent rockets towards Jerusalem, but they were intercepted to the west of the city by Israel’s Iron Dome defence system. Rockets fired towards Tel Aviv, Israel’s Mediterranean business hub, on Saturday were also intercepted.

A spokesperson for Israel’s military said that, as of Sunday morning, 580 rockets had been fired at Israel, adding that 97 per cent of the 450 that had reached Israeli skies had been intercepted.

The fighting in Gaza has sparked concerns that Hamas, which has run the Gaza Strip since 2007 and has greater military capabilities than Islamic Jihad, could enter the fighting. Israel and Hamas have fought four wars in the past 15 years.

As of Sunday Hamas had not joined, although its spokesman Hazem Qasem issued a statement saying Israel was “deluded” in thinking it could “undermine the growing resistance across Palestine by assassinating Palestinian resistance commanders”.

The conflict has further strained Gaza’s already ravaged economy. Israel closed border crossings into the strip last week for fear of reprisals following its arrest of Bassam al-Saadi, a senior Islamic Jihad official, in the West Bank.

The closures have prevented fuel deliveries to Gaza’s only power plant, which stopped functioning on Saturday, exacerbating the power cuts that already plague the territory.