Israel resealed border crossings with the Gaza Strip on Tuesday, blaming continued rocket fire at its towns, despite warnings from world aid groups of looming shortages of food and fuel in the coastal territory.

Israel had allowed 33 truckloads of supplies into Gaza for the first time in two weeks on Monday, and Israeli Prime Minister Ehud Olmert told Palestinian President Mahmoud Abbas he would not permit a humanitarian crisis to develop there.

"The crossings are shut because of ongoing rocket fire," said Peter Lerner, a defense ministry spokesman, referring to several barrages of rockets fired from Gaza on Monday that slammed into Israeli towns but caused no injury.

A statement issued later on Tuesday said Defence Minister Ehud Barak had ordered the crossings to remain closed on Wednesday "following continued rocket fire towards Israel."

International aid groups said the supplies sent in on Monday were not enough to alleviate food shortages.

Israel has also held up fuel shipments to Gaza's main power plant, leading to periodic electricity blackouts each day for many of the 1.5 million Palestinians living in the territory.

Until Monday, Israel had not allowed UNRWA, a United Nations agency that aids some 750,000 refugees in Gaza, to bring in supplies since November 4 during cross-border fighting in which more than a dozen Palestinian fighters were killed.

Several Israelis have been lightly wounded by dozens of rockets fired by gunmen after Israeli raids.

Hamas gunmen fired mortar bombs at Israeli soldiers searching for explosives near the Gaza border fence on Tuesday, the Israeli military and Hamas said. There were no reported casualties from that incident.

"MASSIVE SUFFERING"

U.N. High Commissioner for Human Rights Navi Pillay called for an immediate end of the Israeli blockade of Gaza, which she said was "in direct contravention of international human rights and humanitarian law."

"It must end now," she said in a statement released in Geneva on Tuesday. "Only a full lifting of the blockade followed by a strong humanitarian response will be adequate to relieve the massive humanitarian suffering evident in Gaza today."

The Israeli Foreign Ministry said in a statement that Pillay's assessment of the humanitarian situation in the Gaza Strip was "one-sided" and based on "misinformation" provided by Hamas, which controls the territory.

"Overall responsibility for the situation in the Gaza Strip lies with Hamas ... It is disappointing to see the High Commissioner fall victim to Hamas' cynical manipulation of the media," the statement said.

The British-based Oxfam International humanitarian agency said in a statement that "only the bare minimum of goods have entered Gaza in the past couple of days."

Barbara Stocking, Oxfam's chief executive, said the group "fears a serious worsening once again of the humanitarian situation if urgent action is not taken."

In talks with Olmert in Jerusalem on Monday, Abbas urged Israel to abide by a 5-month-old Egyptian-brokered truce with Hamas Islamists, a deal that has neared collapse in the past two weeks of fighting.

Abbas, involved in peace talks with Israel since last year which Hamas rejects, has condemned Israel's tightened blockade of Gaza as a "war crime."

Olmert promised Abbas in talks they held on Monday that Israel would free some 250 Palestinian prisoners to the occupied West Bank, out of some 11,000 whom Israel holds in its jails.

Israeli troops arrested 32 more Palestinian suspects in overnight raids against militant hideouts on Tuesday in the West Bank, a military spokesman said.