In a deadly escalation that has raised fears of war, Israel launched waves of retaliatory strikes against the Gaza Strip on Sunday after Palestinian rockets hit Israeli cities.

Gazan authorities said 23 Palestinians, including at least six militants, were killed by Israeli strikes in fighting that began Saturday with massive rocket fire from the strip.

However, Israel challenged their claim that a pregnant woman and a baby had been killed by Israeli forces and blamed their deaths on errant fire from Hamas, the Islamist movement that rules the enclave.

Four people were killed in Gaza rocket and missile strikes on southern Israel on Sunday. Three were confirmed as Israeli, the army said.

The violence erupted as Hamas, the Islamist movement that rules the enclave, sought further steps from Israel toward easing its blockade under a fragile months-old ceasefire.

Nineteen Palestinians were killed on Sunday, including a commander for Hamas’s armed wing who Israel said it targeted due to his role in transferring money from Iran to militant groups in the Gaza Strip.

It was a rare admission of a targeted killing by Israel’s army.

Israeli Prime Minister Benjamin Netanyahu said on Sunday morning he instructed the military “to continue its massive strikes on terror elements in the Gaza Strip.”

He said he had also ordered reinforcements near Gaza.

Hamas leader Ismail Haniya said in a statement on Sunday night that “returning to a state of calm is possible” if Israel committed to a “complete ceasefire.”

Without it, “the arena could face many rounds of confrontation,” he said.

Rocket fire and Israeli strikes continued into Sunday evening.

Israel said its strikes were in response to Hamas and Islamic Jihad firing more than 600 rockets or mortars across the border since Saturday, with Israeli air defenses intercepting more than 150.

In addition to those killed and injured, the rockets repeatedly set off air raid alarms in southern Israel and sent residents running to shelters while also damaging houses. At least 35 of the rockets fell in urban areas, according to the army.

The army said its tanks and planes hit some 320 militant targets in Gaza in response.

It targeted militant sites and in some cases militants themselves as well as their homes if they were found to be storing weapons, military spokesman Jonathan Conricus said.

Several buildings in Gaza City were destroyed.

Israel said one of the buildings included Hamas military intelligence and security offices.

Turkey said its state news agency Anadolu had an office in the building, and strongly denounced the strike.

Israel said another destroyed building housed Hamas and Islamic Jihad offices.

A man with the body of a loved one in a hospital morgue, after Israeli warplanes carried out airstrikes over Beit Lahia, Gaza, on Sunday. Photo: Ramez Habboub / Anadolu Agency

The Gaza Health Ministry said the dead from the Israeli strikes included a 14-month-old baby and a pregnant woman, 37. It first identified the woman as the baby’s mother, but the family clarified on Sunday that she was the aunt.

Conricus said that based on intelligence “we are now confident” that the deaths of the woman and baby were not due to an Israeli strike.

“Their unfortunate death was not a result of [Israeli] weaponry but a Hamas rocket that was fired and exploded not where it was supposed to,” he said.

The Gazan ministry reported late Sunday that another four-month-old baby was among those killed in Israeli strikes in the northern Gaza Strip. Israel’s army had no comment.

Islamic Jihad’s armed wing distributed a video showing militants handling rockets and threatening key Israeli sites, including Ben-Gurion international airport near Tel Aviv.

On Sunday, Hamas and Islamic Jihad said their armed wings had targeted an Israeli army vehicle with a Kornet missile.

Conricus said a Kornet missile had hit a vehicle and killed an Israeli civilian.

Israel closed its crossings with Gaza for people and goods, as well as the fishing zone off the enclave’s shore, until further notice.

Egyptian and UN officials held talks to calm the situation, as they have done repeatedly in the past, while the European Union called for an immediate halt to rocket fire from Gaza.

The United States said it fully supported Israel’s “right to self-defence against these abhorrent attacks.”

Deploring the “risk of yet another dangerous escalation and further loss of life on the eve of the holy month of Ramadan,” UN Secretary-General António Guterres urged all parties to “exercise maximum restraint, immediately de-escalate and return to the understandings of the past few months.”

The UN’s special coordinator for this conflict’s peace process, Nickolay Mladenov, issued a similar call for calm on Saturday and continues to work closely with Egypt and all concerned parties to restore calm, UN News reported.

– with reporting by AFP, Haaretz and UN News