Israeli tanks massed at the Gaza border on Monday as warplanes continued pounding Hamas targets in the densely populated enclave where raids have killed nearly 300 people in two days.

An Israeli soldier stands atop a tank near Kibbutz Sufa, just outside the southern Gaza Strip December 28, 2008. [Agencies]

Dozens of tanks and personnel carriers idled at several points near the border after Israel warned it could launch a ground offensive in addition to its massive air blitz.

Hamas responded to the continuing bombardment by firing rockets the farthest yet into Israel, with one striking not far from Ashdod, Israel's second-largest port, some 30 kilometres (18 miles) north of Gaza . It caused no casualties, medics said.

The Islamist movement accused Israel of "committing a holocaust as the whole world watches and doesn't lift a finger to stop it."

Hamas spokesman Fawzi Barhum said the movement "reserves the right to hit back at this aggression with martyr operations," meaning suicide bombings of the sort Hamas has not carried out inside Israel since January 2005.

Britain, France and Russia joined the growing international chorus for a halt to the violence.

Israeli Defence Minster Ehud Barak vowed to "expand and deepen" the bombing blitz, which was unleashed in retaliation for persistent rocket fire by militant groups.

"If it's necessary to deploy ground forces to defend our citizens, we will do so," his spokesman quoted him as saying.

The cabinet gave the green light to call up 6,500 reserve soldiers, a senior official told reporters after the meeting.

Warplanes continued to pound the impoverished and overcrowded territory of 1.5 million people, where many streets were deserted, and schools and shops stayed shut as hundreds of funerals were held.

Jets bombed a series of tunnels on Gaza's border with Egypt -- a lifeline used by Hamas to smuggle goods and weapons into the enclave, which has been virtually sealed off by Israel since the Islamists seized power in June 2007.

Later on Sunday, jets targeted several metal workshops, which the Israeli army said were being used to manufacture makeshift rockets.

Another strike on a mosque in the Jabaliya refugee camp north of Gaza City left four dead, including a four-year-old girl, according to the head of Gaza's emergency services Moawiya Hassanein.

One woman and a man were also killed when a missile hit a family home in the neighbourhood of Zeitoun in eastern Gaza City, medics said.

Aircraft also bombed Gaza City's Islamic University early on Monday, a stronghold symbol of Hamas. Fire and smoke billowed from the building which was hit by at least five missiles, according to witnesses.

And as pressure mounted within the impoverished territory, dozens of Gazans tried to break through the border into Egypt on Sunday, only to be stopped by Egyptian police firing into the air.

An Egyptian policeman was killed and another wounded by shots from across the border in the divided town of Rafah, a security official and medics said, adding that the source of the fire was unclear.

A Palestinian human rights group branded the Israeli pounding of Gaza one of the bloodiest days in Israel's 40-year occupation. As well as nearly 300 dead, more than 600 people have been wounded, medics said.

Israeli Prime Minister Ehud Olmert said the campaign was launched "in order to regain a normal life for the citizens in the south who have suffered for many years from incessant rocket, mortar and terror attacks."

Foreign Minister Tzipi Livni urged the international community to put the blame squarely on Hamas.

"I expect the international community, including the entire Arab world, to send a clear message to Hamas: 'It is your fault. It's your responsibility. You're the one who's being condemned,'" she told US television network NBC.

The Israeli bombardment has sparked widespread international concern.

In New York, the UN Security Council called for an "immediate halt to all violence."

Egypt, which brokered a six-month truce between Israel and Hamas that expired on December 19, said it was trying to negotiate a new ceasefire.

And the UN envoy to the Middle East Robert Serry called for a new truce with international backing, saying that there was no military solution to the conflict.

But a senior Israeli official insisted: "We have our goals and our timetable, and we don't seek mediation."

Israel in the meanwhile announced it would allow 100 truck-loads of humanitarian aid into Gaza on Monday but said it would maintain its crippling blockade on the impoverished territory.

Israel's main ally Washington blamed Hamas "thugs" for provoking the offensive by firing rockets into the Jewish state from Gaza, and urged Israel to avoid causing civilian casualties.

The Israeli offensive sparked protests across the world. In the occupied West Bank, two demonstrators were killed in clashes with police.

Israel unleashed "Operation Cast Lead" against Hamas in the middle of Saturday morning, with some 60 warplanes hitting more than 50 targets in just a few minutes.

By Sunday, some 230 targets had been hit, the military said.

Hamas has responded by firing more than 90 rockets and mortar rounds at Israel, killing one man and wounding some 20 people.

The Israeli blitz came after days of spiralling violence since the expiry of the Gaza truce. It comes less than two months before snap parliamentary elections in Israel called for February 10.