A 72-hour ceasefire took effect today in Gaza but was immediately threatened by deadly shelling, as diplomats pressed for a more durable end to 25 days of devastating violence.

The skies over Gaza initially fell silent after the humanitarian truce started at 8am local time, the latest and longest of several agreed since the conflict broke out on July 8.

It gave a brief respite to people in the battered strip, after more than three weeks of fighting that has killed 1,459 on the Palestinian side, mostly civilians, and 61 soldiers and three civilians on the Israeli side.

Palestinian medics said Israeli shelling killed eight people in southern Gaza, but Israel accused Gaza militants of breaking the nascent ceasefire hours after it began.

US Secretary of State John Kerry had said earlier that once the ceasefire was under way, Israeli and Palestinian representatives, including from Hamas, would begin talks in Cairo on a more durable truce.

The ceasefire was a joint US-UN initiative and would give civilians "a much needed reprieve", Kerry said in New Delhi.

"This is a respite, a moment of opportunity – not an end. It's not a solution," he warned, saying Israeli forces would remain inside Gaza and to carry out "defensive" operations to destroy tunnels used to attack its territory.

As the truce began, Gaza doctor Belal al-Dabour tweeted: "This ceasefire should give us a glimpse on our life for the coming months after Israel destroyed everything. It's not going to be pretty."

But within hours an AFP journalist on the Israeli side of the border heard air raid sirens warning of a rocket attack, and the sound of outgoing Israeli artillery fire.

Medics said eight people were killed in bombardment east of the town of Rafah, in southern Gaza, as heavy shelling of the area was heard.

Israel immediately accused Hamas and other Gaza militants of "flagrantly violating" the truce.

"Once again the terror organisations in Gaza flagrantly violating the ceasefire to which they committed themselves, this time to the US Secretary of State and the UN Secretary General," Prime Minister Benjamin Netanyahu's office said, without pointing to a specific incident.

Israeli tank and air fire killed 14 Palestinians in Gaza prior to Friday's ceasefire deadline, and the army said five of its soldiers died in mortar fire near the border with the Palestinian coastal enclave.

While the ceasefire had been accepted in the name of all militant groups by Hamas, the main power in Gaza, the Islamist movement stressed it was dependent on Israel reciprocating.

"Hamas and all the resistance movements have accepted a 72-hour humanitarian ceasefire from 8:00 am Friday which will be respected by all these movements if the other party also observes the ceasefire," said spokesman Fawzi Barhum.

A source in Netanyahu's office said: "Israel has accepted the US/UN proposal for a 72-hour humanitarian ceasefire beginning 8am Friday".

Hamas allies Turkey and Qatar welcomed the ceasefire, as did British Foreign Secretary Philip Hammond.

"We should now redouble our efforts and leave no stone unturned, to ensure this is a lasting and durable ceasefire to make way for substantial discussions to resolve the underlying issues on both sides," said Hammond.

Only minutes before the deadline, Palestinians had continued to fire rockets into southern Israel, with five brought down by missile defences, army radio said.

The Israeli army said that "five soldiers were killed during operational activity along the border with the Gaza Strip when a mortar was fired at the forces."

The ceasefire came after the UN Security Council expressed "grave disappointment" that repeated calls for a truce had not been heeded, and demanded a series of humanitarian breaks to ease conditions for civilians trapped in the war-torn territory.

Egypt has invited Israel and the Palestinian Authority to send delegates to Cairo for longer-term truce talks.

"Egypt emphasises the importance of both sides committing to the ceasefire so the negotiations can take place in a favourable atmosphere," the foreign ministry in Cairo said.

The delegations were expected to start arriving in Cairo later on Friday.

Deputy Secretary of State William Burns, the number-two US diplomat, will attend in the hope of extending the truce beyond 72 hours, a senior US official said.

The announcement came as the White House said there was little doubt that Israeli artillery was the source of a "totally indefensible" strike on a UN school in northern Gaza that killed 16 people on Wednesday.

The school was sheltering more than 3,000 Palestinians made homeless by the relentless fighting.

"It does not appear there's a lot of doubt about whose artillery was involved in this incident," spokesman Josh Earnest said.

The Israeli army has suggested the deaths may have been the result of a misfired Palestinian rocket.
(AFP)