Food distribution to half the Gaza Strip's 1.5 million population resumed on Tuesday, although the United Nations warned aid supplies would soon run out unless Israel eases its crippling blockade.

"Distribution will go on of the very small amount we brought in on Monday," said UN Relief and Works Agency spokesman Chris Gunness.

"The supplies will last days, not weeks," he told AFP.

Crowds rushed to the UNRWA distribution centres to try to get hold of the limited supplies of flour, sugar, rice, powdered milk and luncheon meat.

"I can't wait to receive the aid. Our lives are in ruins," said Um Said, 60, who with her husband looks after 15 children and grandchildren.

On Monday, the first shipment of supplies in two weeks made it possible to resume food distribution after a four-day interruption, but Israel again sealed off the Palestinian territory on Tuesday, citing continuing violence.

Defence Minister Ehud Barak ordered Israel's crossings to remain closed on Wednesday after militants in the Hamas-ruled territory continued to fire rockets against southern Israel on Tuesday, a defence official said.

Israeli armoured vehicles came under mortar attack as they rolled into southern Gaza for a routine search for explosive devices along the border fence, a military spokesman said.

Hamas, the Islamist group that rules Gaza, claimed it fired a rocket at the vehicles.

Later, Gaza militants fired three rockets that exploded in open areas in southern Israel, causing no casualties or damage, the army said.

Israel responded to a recent surge in rocket and mortar attacks by completely sealing off Gaza on November 5. That prevented delivery of the basic supplies that had been allowed in under a blockade imposed after Hamas seized control of the territory in June 2007.

Last week, amid mounting pressure from the international community, Israel allowed some industrial fuel to be delivered to Gaza's sole power plant. And on Monday it let in 33 truckloads of humanitarian and other basic supplies.

UNRWA, which feeds 750,000 people in the impoverished sliver of land, said thousands of dollars worth of powdered milk were lost after Israeli officials slashed the packages for inspection.

"Babies should not be punished by being deprived of milk. I am not aware of babies firing rockets or baby milk being used to power rockets," said Gunness.

He also said food would run out in days unless new supplies are allowed in, but Israeli officials indicated the crossings were to remain closed on Tuesday.

"This decision has been taken by Defence Minister Ehud Barak because of the continued Palestinian rocket fire at southern Israel," said Peter Lerner, an Israeli military spokesman.

At least six rockets were fired at southern Israel on Monday, without causing casualties.

The Israeli parliament is to return from its winter recess to hold a special session on the Gaza violence next Monday, a spokesman said. The session was called at the request of the right-wing opposition.

Nearly every day for the past two weeks, Israeli forces and Gaza militants have engaged in tit-for-tat attacks, which have left 15 Palestinian gunmen dead.

Israel had been expected to ease its blockade after a truce went into effect on June 19, but says continuing rocket and mortar attacks make this impossible.

Hamas accuses it of failing to deliver on its side of the bargain.

The top UN human rights official called from Geneva for Israel to immediately lift its blockade, saying it violated international law.

"By function of this blockade, 1.5 million Palestinian men, women and children have been forcibly deprived of their most basic human rights for months," said the High Commissioner for Human Rights, Navi Pillay.

"This is in direct contravention of international human rights and humanitarian law. It must end now," she added.

The Israeli mission in Geneva condemned Pillay's call as "utterly shortsighted" and argued that she had not addressed the issue of rocket fire targeting Israeli civilians.