GAZA, Dec 21 (Reuters) - Fighting in the Gaza Strip escalated on Thursday (Dec 21) with some of the most intense Israeli bombardment of the war and Hamas demonstrated its ability to rocket Tel Aviv, even as the foes engaged in the most serious talks for weeks on a new truce.

Israeli bombing was at its most intense over northern Gaza, where orange flashes of explosions could be seen from across the fence in Israel in the morning hours. Later, Israeli planes roared over central and southern areas, dropping bombs that sent up plumes of smoke, residents said.

In Israel's commercial capital Tel Aviv, sirens wailed and rockets exploded overhead, intercepted by Israeli defences. Shrapnel fell on a school but the children were in shelters and there were no reported casualties, Israel's Ynet news site said.

The armed wing of Hamas said it had fired the salvo in response to the Israeli killing of civilians. But with the group's leader in Cairo for truce talks, the attack seemed timed to send a message that nearly 11 weeks of war had failed to destroy the militants' strike capability.

Both sides remained far apart in public. Israeli Prime Minister Benjamin Netanyahu vowed again to fight on until the eradication of Hamas, the militant group that sent fighters over the border into southern Israel on Oct 7, taking some 240 hostages and killing 1,200 people.

"Surrender or die," he told Hamas in a video address.

Hamas said Palestinian factions had taken a united position that there should be "no talk about prisoners or exchange deals, except after a full cessation of (Israeli) aggression".

Residents in Jabalia in the north of the Strip close to the Israeli border said the area was completely cut off, with Israeli snipers now firing on anyone trying to escape.

"It was one of the worst nights in terms of the occupation bombings," said one Jabalia resident who asked not to be identified for fear of reprisal.

Nearly 20,000 Gazans have been confirmed killed since the start of the conflict, according to the Palestinian health ministry, with several thousand more bodies believed trapped under rubble. Nearly all of Gaza's 2.3 million people have been driven from their homes.

A report by a UN-backed body said the entire population of Gaza is facing crisis levels of hunger. The risk of famine is increasing each day, added the Integrated Food Security Phase Classification (IPC).

"This report sort of confirms our worst fears," said Arif Husain, chief economist and director of research at the UN World Food Programme.

"I've been doing this for the last 20 plus years. I've been to Afghanistan, I've been to Yemen, to Syria, South Sudan, Ethiopia, northeast Nigeria. But I've never seen something this bad happening this quickly," he told Reuters in an interview.

By the afternoon, Israel intensified bombing of the Gaza City suburb of Sheikh Radwan, residents said. Hamas and the Islamic Jihad militant group said they fired rockets and mortar bombs at Israeli forces massing on the Gaza side of the border. Reuters could not confirm the battlefield reports.

The Palestinian Red Crescent said it had reports Israeli forces had stormed an ambulance centre in Jabalia and arrested paramedics. The Israeli military said it needed more details on the report to comment and was following international law and taking "feasible precautions to mitigate civilian harm".

The World Health Organization said the last hospital in the northern half of the Gaza Strip had effectively ceased functioning over the past two days, leaving no place left to take the wounded.

Photo from Reuters