A Palestinian boy walks over debris as civilians who were displaced from their houses due to fighting between Israel's army and Hamas fighters return to check their homes in Gaza City's Shejaiya neighborhood, on Friday. Photo: AFP

Israel declared a Gaza ceasefire over on Friday only hours later and killed more than 50 Palestinians in renewed shelling, saying militants had breached the truce shortly after it began and apparently captured an Israeli soldier.

The 72-hour pause in hostilities announced by US Secretary of State John Kerry and UN Secretary-General Ban Ki-moon was the most ambitious attempt so far to end more than three weeks of fighting, and followed mounting international alarm over a rising Palestinian civilian death toll.

The ceasefire was to be followed by Israeli-Palestinian negotiations in Cairo on a longer-term solution.

A joint Palestinian delegation, including Hamas and Islamic Jihad, is to travel to Cairo on Saturday for ceasefire talks, President Mahmud Abbas's office announced.

"Abbas has formed the delegation, which will head Saturday for Cairo whatever the circumstances," his office said in a statement.

The Israeli military said that 90 minutes into the truce, militants attacked soldiers searching for infiltration tunnels in the southern Gaza Strip.

"Out of a tunnel access point or several, terrorists came out of the ground. There was an exchange of fire," said Lieutenant-Colonel Peter Lerner, a military spokesman.

Two of the soldiers were killed during the exchange.

Asked if the ceasefire was over, Lerner replied: "Yes. We are continuing our activities on the ground." He said Israeli forces were mounting an "extensive effort" to locate the soldier.

The US on Friday strongly blamed Hamas for violating a 72-hour ceasefire in Gaza.

Kerry called on Hamas to immediately and unconditionally release the missing Israeli soldier.

The Gaza Health Ministry said more than 50 people were killed and 220 wounded by Israeli shelling after the incident near the southern town of Rafah.

There was no immediate word from militant groups on whether any were holding the soldier, identified by the military as Second Lieutenant Hadar Goldin, 23. Sami Abu Zuhri, a spokesman for the dominant Hamas movement in Gaza, said Israel was trying to mislead the world and "cover up its Rafah massacre."

Israel launched its offensive in Gaza on July 8, unleashing air and naval bombardments in response to a surge of cross-border rocket attacks. Tanks and infantry pushed into the territory of 1.8 million on July 17.

Gaza officials say at least 1,509 Palestinians, mostly civilians, have been killed and 7,000 wounded. Sixty-three Israeli soldiers have been killed and more than 400 hurt. Three civilians have been killed by Palestinian rockets in Israel.

Eight rockets and mortar bombs were fired from Gaza at Israel after the ceasefire began, the military claimed.

The military added that one was intercepted by the Iron Dome system and seven hit open areas.

Amid strong public support in Israel for the Gaza campaign, Prime Minister Benjamin Netanyahu had faced intense pressure from abroad to stand his forces down.

Hamas, isolated in an Arab world concerned about the rise of Islamist militancy, is seeking an end to Israel's blockade of Gaza.

It also wants a hostile Egypt to ease restrictions at its Rafah crossing with the territory, imposed after the Egypt's military toppled Islamist president Mohamed Morsi last July.

A senior State Department official traveling with Kerry in India had said US Deputy Secretary of State Bill Burns would arrive in Cairo on Saturday and that Frank Lowenstein, the acting US envoy for Middle East peace, would be there on Friday.

US officials said Israel and the US would not sit across the table from Hamas, which the two countries, along with the EU, consider a terrorist group.

The UN on Friday also urged Palestinian parties to reaffirm their commitment to a 72-hour Gaza ceasefire.

UN Special Coordinator Robert Serry "urges the Palestinian parties to last night's understanding to urgently reaffirm their commitment to the humanitarian ceasefire," a statement said.