GAZA CITY, Palestinian Territories (AFP) - A Gaza truce set to last for three days collapsed only hours after it began on Friday as Israel pounded Palestinian militants in apparent response to renewed rocket fire, jeopardising international efforts for a durable ceasefire.
The skies over Gaza initially fell silent after the humanitarian truce announced overnight by US Secretary of State John Kerry, the longest of several agreed since the conflict broke out on July 8.
It gave a brief respite to people in the battered strip from fighting that has killed nearly 1,500 on the Palestinian side, mostly civilians, and 61 Israeli soldiers and three civilians on the other.
But within hours air raid sirens warning of rocket fire were heard on the Israeli side of the border, and heavy artillery shelling renewed in the southern city of Rafah, killing at least 27 people and injuring 100 more, medics said.
AFP correspondents said there appeared to be fierce fighting ongoing in the vicinity of Rafah, and medics had trouble retrieving the dead and wounded.
The army warned residents of the city to remain in their homes.
"The residents of Rafah must stay in their houses. The army is pursuing terrorist elements in Rafah," a voice message sent to cellphones of Gaza residents said.
Mr 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", the top US diplomat 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.
But a few hours after the truce began at 5am GMT, Israel accused Hamas and other Gaza militants of "flagrantly violating" the ceasefire.
"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.
Both Hamas and Israel issued statements saying they accepted the 72-hour humanitarian truce.
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 Mr 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.