Saturday was one of the deadliest days since the US-brokered truce between Israel and Hamas came into effect on October 10, after two years of war.

The Israeli military said an "armed terrorist" had crossed the so-called Yellow Line within the Gaza Strip, behind which Israeli forces have withdrawn, and fired at Israeli soldiers.

In response to the incident in southern Gaza, which it said was on a route used for humanitarian aid deliveries in the territory, the Israeli military said it "began striking terror targets in the Gaza Strip".

Mahmud Bassal, spokesman for the civil defence agency which operates under Hamas authority, told AFP there were "21 martyrs this evening in five separate Israeli air strikes, in a clear violation of the ceasefire in Gaza".

They included seven killed and more than 16 injured in a strike on a house in Nuseirat in the central Gaza Strip, and four killed and several injured in an air strike on a residential apartment in the Al-Nasr district, west of Gaza City, he said.

According to the health ministry in Hamas-run Gaza, as of Thursday, 312 Palestinians had been killed by Israeli fire since the truce took hold.

- Children carried into hospital -

Bassal said one strike hit a house in Deir el-Balah in the central Gaza Strip.

At Al-Aqsa hospital in Deir el-Balah, AFP witnessed children being brought to the hospital in ambulances and carried inside. Some casualties were taken in on stretchers.

Health ministry spokesman Khalil al-Daqran said "more than 20 injured" had been brought to the hospital, "most of them women and children".

"Among the injuries, there are many severe wounds to the head and chest," he told AFP.

The first reported strike targeted a vehicle in the Al-Rimal neighbourhood in western Gaza City. Five people were killed and several injured, said Bassal.

An AFP photographer at the scene witnessed passers-by approaching the wreckage of the burnt-out car, with children appearing to be trying to salvage food from inside.

-Truce violation claims -

Israeli Prime Minister Benjamin Netanyahu's office accused Hamas of breaching the truce agreement.

"Hamas violated the ceasefire again, sending a terrorist into Israel held territory to attack IDF soldiers," it said on X. "In response, Israel eliminated five senior Hamas terrorists."

"Israel has fully honoured the ceasefire, Hamas has not. We again call on the mediators to insist that Hamas fulfil its side of the ceasefire."

In a statement, Hamas said the "escalation" of Israeli violations were "attempts to undermine the ceasefire".

"We call on the mediators to intervene urgently and exert pressure to stop these violations immediately."

The Palestinian foreign ministry, based in the Israeli-occupied West Bank, condemned the strikes.

It urged the international community to put "immediate pressure" on Israel in order to "stop the massacres".

- Ceasefire 'pointless' -

The UN humanitarian agency OCHA said the ceasefire scale-up of aid deliveries into Gaza "is still being held back by restrictions affecting visas and import approvals, too few crossing points operating" and other impediments.

Jihad Abed Al-Aziz, who was displaced to Khan Yunis in southern Gaza, was at a food distribution point where dozens of people jostled for a pan of rice.

"A ceasefire is pointless," the 55-year-old told AFP.

"The crossings do not bring in enough to give us food, supplies, or even the basics of life.

"We have lost our jobs, our homes, and everything in our lives. Life itself has no meaning any more."

The war was sparked by Hamas's October 7, 2023 attack on Israel, which resulted in the deaths of 1,221 people.

Israel's retaliatory assault on Gaza has killed at least 69,733 people, according to figures from the health ministry that the UN considers reliable.