Israel said it had resumed enforcing a Gaza ceasefire after carrying out dozens of deadly strikes on Hamas positions on Sunday, having accused the group of targeting its troops in the most serious violence since the nine-day-old truce began.

Gaza's civil defence agency, which operates under Hamas authority, said at least 45 people had been killed across the territory in Israeli strikes.

Four hospitals in Gaza confirmed the death toll of 45 to AFP, saying they had received the dead and wounded.

Israel's military said it was looking into the reports of casualties.

Later on Sunday, the military said it had "renewed enforcement of the ceasefire" but vowed to "respond firmly to any violation of it".

The strikes began in response to what the Israeli military said in a statement had been a "blatant violation" of the ceasefire.

It said it struck Hamas targets across the enclave, including field commanders, gunmen, a tunnel and weapons depots, after militants launched an anti-tank missile and fired on its troops, killing the soldiers.

At least one strike hit a former school sheltering displaced people in the area of Nuseirat, residents said.

The Islamist group denied the accusations, one official accusing Israel of fabricating "pretexts" to resume the war.

In a separate statement, the Israeli military said two of its soldiers "fell during combat in the southern Gaza Strip". The prime minister's office later said they were killed in the city of Rafah.

A security official also told AFP that Israel was suspending the entry of aid into Gaza due to ceasefire violations. However, an Israeli security source told Reuters that aidset to resume on Monday following US pressure.

Israel repeatedly cut off aid to the territory during the war, exacerbating dire humanitarian conditions, with the United Nations saying it caused a famine in northern Gaza.

The truce in the Palestinian territory, brokered by US President Donald Trump and taking effect on 10 October, halted more than two years of devastating war between Israel and Hamas.

The deal established the outline for hostage and prisoner exchanges, and was proposed alongside an ambitious roadmap for Gaza's future. But it has quickly faced challenges to its implementation.

Defence Minister Israel Katz warned Hamas would "pay a heavy price for every shot and every breach of the ceasefire", adding Israel's response would "become increasingly severe".

A statement from Izzat Al-Rishq, a member of Hamas's political bureau, reaffirmed the group's commitment to the ceasefire.

Israel, it said, "continues to breach the agreement and fabricate flimsy pretexts to justify its crimes".

Hamas's armed wing insisted on Sunday that the group was adhering to the ceasefire agreement with Israel and had "no knowledge" of any clashes in Rafah.

Under Trump's 20-point plan, Israeli forces have withdrawn beyond the so-called Yellow Line. That leaves them in control of around half of Gaza, including the territory's borders but not its main cities.

Hamas in turn has released 20 surviving hostages and is in the process of returning the remaining bodies of those who have died. (Agencies)