"It was an outrageous violation of the ceasefire negotiated over the past several days, and of the assurances given to the US and the United Nations," John Kerry, US Secretary of State, 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, Xinhua quoted Kerry as saying.
US condemned the killing of Israeli soldiers and apparent abduction of another
Kerry urged militants of Hamas, which controls the Gaza Strip, to "immediately and unconditionally" release the missing Israeli soldiers. He also called on those with influence over Hamas to reinforce this message.
"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," Kerry said.
The top US diplomat said he had been in close touch with the Israeli Prime Minister Benjamin Netanyahu, UN special coordinator Robert Serry and other regional partners.
On Thursday, Kerry and the UN chief Ban Ki-moon announced the "unconditional humanitarian ceasefire" deal in Gaza, which was set to commence at 8 a.m. local time Friday and would last for 72 hours unless extended.
The newly reached truce collapsed soon after it came into effect Friday morning, with Israel resuming full operations in the Gaza Strip following an attack that reportedly left two soldiers dead.
Earlier Friday, Ban also condemned "in the strongest terms" the Hamas violation of the truce, urging Israelis and Palestinians to exercise maximum restraint and return to the ceasefire.