Israel and Hamas have agreed to a 72-hour ceasefire in the Gaza conflict, US Secretary of State John Kerry and the United Nations announced early Friday.

Kerry said the two sides would begin the truce at 8 am local time (0500 GMT) and that Israelis and Palestinians would enter talks in Cairo.

The top US diplomat said the ceasefire would last for 72 hours "unless extended."

"During this time the forces on the ground will remain in place," Kerry said in a joint statement with UN Secretary General Ban Ki-moon, who has also been shuttling around the Middle East.

"This ceasefire is critical to giving innocent civilians a much-needed reprieve from violence," the statement said.

But Kerry, who aides said has held some 100 phone calls on the crisis in the past week despite a trip to India, warned that the ceasefire was not final.

"I want to emphasise -- this is not a time for congratulations and joy, or anything except a serious determination, a focus by everybody to try to figure out the road ahead," Kerry told reporters in an early morning news conference on a visit to New Delhi.

"This is a respite, a moment of opportunity -- not an end. It's not a solution. It's the opportunity to find a solution," Kerry said.

A US official, speaking on condition of anonymity, said that Israeli and Palestinian officials, including Hamas representatives, would head to Cairo as soon as Friday to launch talks.

Palestinian president Mahmud Abbas, who comes from a rival faction, will name Hamas representatives but the militant group will not directly engage with Israel or the United States, the official said.

Kerry said Israel would still be allowed to carry out "defensive" operations behind what it sees as its lines in Gaza to destroy tunnels. Israel has vowed to wipe out tunnels through which Hamas militants can infiltrate it for attacks.

"During this period, civilians in Gaza will receive urgently needed humanitarian relief, and the opportunity to carry out vital functions, including burying the dead, taking care of the injured, and restocking food supplies," Kerry and Ban said in their statement.

"Overdue repairs on essential water and energy infrastructure could also continue during this period."

Kerry said he had been working the phones with the Middle East from New Delhi in hopes of brokering a halt to the conflict, which has killed 1,435 people over 24 days, largely Palestinian civilians.

The announcement of the truce came even though the Israeli army has mobilised another 16,000 reservists, hiking the total number of soldiers called up to 86,000.

Israel does not say how many troops are currently fighting inside the Gaza Strip.

The UN Security Council has called for humanitarian pauses in Gaza and renewed its appeal for an immediate ceasefire, even though repeated appeals for a truce had gone unheeded.

Earlier, Kerry was tight-lipped as he went ahead with a busy schedule in New Delhi, which included touring the prestigious Indian Institute of Technology to chat to graduate students about environmental innovations.

Speaking to reporters earlier Thursday with his Indian counterpart Sushma Swaraj, Kerry said he remained "hopeful" about a ceaesefire but declined to specify any timeframe.

Kerry has come under fire from hawkish elements in Israel accusing him of yielding too much to Hamas in his negotiations to end the violence.

US President Barack Obama has repeatedly voiced concern to Israeli Prime Minister Benjamin Netanyahu about the heavy toll on civilians, while also defending the US ally's right to defend itself against Hamas.

*This story was edited by Ahram Online