The 72-hour cease-fire brokered by the United States and the United Nations ended just hours after it went into effect, as fighting resumed on both sides Friday morning.

Code Red sirens rang out in southern Israel around 10 a.m., just as Palestinian media reports of Israeli artillery fire began to surface.

At least 40 people were killed in the Israeli shelling of southern Gaza since that time, according to the local hospital in Rafah.

Palestinian news agency Ma'an also reported that three other people were shot by IDF sniper fire in the southern Gaza Strip.

An IDF spokesperson said she was looking into the incidents.

Israel Radio reported that over 15 rockets and mortars had been fired into Israel since the fighting resumed, and that seven rockets had been successfully intercepted by Iron Dome.

It was not immediately clear whether the rocket fire or the shelling came first.

Both the Israeli Foreign Ministry and an unnamed Israeli official from inside the Prime Minister's Office blamed Hamas for breaking the cease-fire.

"Once again Hamas has broken its commitment to a cease-fire - a commitment made to John Kerry and the UN Secretary General," said the official.

Reuters contributed to this report.

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