The US on Friday blamed Hamas for the breakdown of a 72-hour humanitarian truce, saying the "barbaric" attack by the Palestinian militant group that left two Israeli soldiers dead in the Gaza Strip was an "outrageous violation" of the ceasefire.

"The United States condemns in the strongest possible terms on Friday's attack, which led to the killing of two Israeli soldiers and the apparent abduction of another. It was an outrageous violation of the ceasefire negotiated over the past several days, and of the assurances given to the United States and the United Nations," US Secretary of State John Kerry said.

"Hamas, which has security control over the Gaza Strip, must immediately and unconditionally release the missing Israeli soldier, and I call on those with influence over Hamas to reinforce this message," he said in a statement After the horrific loss of life in this attack and its aftermath, it would be a tragedy if this outrageous attack leads to more suffering and loss of life on both sides of this conflict, he said.

Kerry said he has been in close touch with Israeli Prime Minister Benjamin Netanyahu, UN Special Coordinator Robert Serry and with other regional partners over the developments.

"The international community must now redouble its efforts to end the tunnel and rocket attacks by Hamas terrorists on Israel and the suffering and loss of civilian life," he said.

Meanwhile, White House Deputy National Security Advisor Tony Blinken told the MSNBC, "This appears to be an absolutely outrageous action by Hamas, using the cover of a ceasefire to conduct a surprise attack through a tunnel, killing Israeli soldiers, and perhaps taking one hostage." "We strongly, strongly condemn it. Israel has the right to defend itself, and it's obviously taking action to do so."

But this is an outrageous action, and we look to the rest of the world to join us in condemning it, and those with influence on Hamas to use that influence, to cease these actions," Blinken said in response to a question.

The humanitarian ceasefire -- brokered by the US and the UN to end more than three weeks of fighting between Israel and Palestinian militant groups in the Gaza Strip -- collapsed just two hours after it began this morning.

Israeli army said two of its soldiers were killed and a third one may have been abducted by militants in the southern Gaza Strip on Friday, raising the death count to 63 soldiers.

Artillery shelling by Israeli in the southern Rafah area on Friday killed at least 50 people, taking the Palestinian death toll to 1,509, mostly civilians, Gaza health officials said.

Israel and Hamas have blamed each other for the collapse of the fragile ceasefire.