US president Barack Obama blamed Hamas for the swift collapse of the latest Gaza ceasefire, accusing the Palestinian group of launching a "barbaric" attack.
President Obama said on Friday it would be "very hard" to get another truce in the Gaza Strip unless Hamas showed it was serious about laying down its arms and could be trusted to keep its word.
"If they are serious about trying to trying to resolve this situation, that soldier needs to be unconditionally released, as soon as possible," Obama told a surprise press conference.
"I think it's going to be very hard to put a ceasefire back together again if Israelis and the international community can't feel confident that Hamas can follow through on a ceasefire commitment."
But Obama also insisted that the deaths of "innocent civilians in Gaza caught in the crossfire have to weigh on our conscience and we have to do more to protect them."
A three-day humanitarian ceasefire in Gaza collapsed only hours after it began amid a deadly new wave of violence.
Intensive shelling killed dozens of people in southern Gaza hours into the short-lived truce on Friday, with Hamas accusing Israel of breaking the ceasefire and Israel saying it was responding to rocket fire.
The skies over Gaza fell silent after the ceasefire announced overnight by US Secretary of State John Kerry, the longest one agreed upon since the conflict began on July 8.
Starting from 0500 GMT, the truce gave brief respite to people in the battered strip from fighting that has killed atleast 1,600 on the Palestinian side, mostly civilians, and 63 Israeli soldiers and three civilians on the other.
Within hours, air raid sirens warning of rocket fire were heard on the Israeli side of the border, and heavy shelling resumed in the southern Gaza city of Rafah, killing at least 35 people and wounding 100, medics said.
| Al Jazeera's Charles Stratford reports from Gaza on scenes of destruction and devastation.
Shortly afterwards the Israeli army said the ceasefire was over and that it was searching for a soldier feared to have been captured in the enclave.
Israeli forces were pressing their "activities on the ground", army spokesman Peter Lerner said, before the military announced that two soldiers had been killed and named the missing man as Second Lieutenant Hadar Goldin, 23.
Israeli Prime Minister Benjamin Netanyahu's office accused Hamas and other Gaza groups of "flagrantly violating" the ceasefire.
Hamas spokesman Fawzi Barhum countered, saying "it is the (Israeli) occupation which violated the ceasefire. The Palestinian resistance acted based on... the right to self defence."
Kerry, fearing an escalation of violence in Gaza, called on Turkey and Qatar to use their influence to secure the release of the Israeli soldier.
Turkey said it would do what it could to help free the Israeli soldier, but stressed that the priority should be the reinstatement of a truce.
"What is important is that the ceasefire is reinstated. To ensure this, together with others, we can take any step that could resolve this Israeli soldier issue. If Turkey can do anything, we will do our best," Foreign Minister Ahmet Davutoglu told reporters on Friday.
"But if the ceasefire is stopped because an Israeli soldier is kidnapped, somebody should account for the 70 Palestinians killed (today). In our eyes, all people are equal," he said.
Cairo truce talks
Egypt, which invited the Israeli government and the Palestinian Authority to come to Cairo for longer-term truce talks, re-iterated later on Friday its invitation to Israeli and Palestinian delegations, saying it is"still in place" despite the 72-hour humanitarian truce breaking down.
Palestinian President Mahmoud Abbas said a joint Palestinian delegation, including Hamas and Islamic Jihad, would travel to Cairo on Saturday for ceasefire talks, despite the renewed fighting in Gaza.
In a speech published after the ceasefire broke down, Saudi King Abdullah hit out at the "inexcusable" world silence over Israel's "war crimes" in Gaza.
"We see the blood of out brothers in Palestine being shed in collective massacres, that have spared nobody, and in war crimes against humanity... all taking place under the eyes and ears of the international community... that has stood indifferently watching events in the whole region," the king said.
"This silence is inexcusable" and will "result in a generation that rejects peace and believes only in violence," he said.
The truce had come after the UN Security Council expressed "grave disappointment" that repeated calls for one had not been heeded, and demanded a series of humanitarian breaks to ease conditions for Gaza's civilians.