Violence flared again around Gaza on Friday, wounding two militants and an Israeli woman and putting a five-month-old truce in jeopardy as UN food handouts to the Hamas-ruled strip ground to a halt.

Israeli Prime Minister Ehud Olmert held emergency talks with Defence Minister Ehud Barak and military top brass following the firing of nearly 20 rockets from Gaza into southern Israel on Friday.

Olmert said at the meeting that the rocket fire was "a blatant and fundamental violation" of the June 19 Egyptian-brokered ceasefire that followed months of deadly clashes between Israel and Hamas.

The Israeli leaders decided to maintain pressure on the Islamist rulers by tightening Israel's blockade on the impoverished territory, according to a statement from his office.

UN Secretary General Ban Ki-moon called on Israel to swiftly allow food aid and fuel supply lines to be opened for Gaza, highlighting the suffering of local people there.

"The secretary-general is deeply concerned at the deterioration of the humanitarian and security situation in Gaza and southern Israel, and at the potential for further suffering and violence. He calls on all parties to uphold international humanitarian and human rights law," a statement from Ban said.

One Israeli woman was wounded by shrapnel when a rocket struck the frequently targeted city of Sderot, police spokesman Micky Rosenfeld said.

At least three rockets hit the city of Ashkelon, home to more than 100,000 Israelis, without causing any casualties, emergency services said, as radio urged residents to remain close to shelters.

Hamas said it had targeted the port city with five Grads -- military-grade missiles that have a longer range than the makeshift weapons usually fired into southern Israel.

In Gaza, two militants were wounded in an Israeli air strike, medics and witnesses said.

Eleven days of tit-for-tat skirmishes have rocked the fragile truce and prompted Israel on November 5 to completely seal off its crossings with Gaza, through which it usually allows a limited quantity of vital goods.

The UN Works and Relief Agency says the closure has forced it to suspend its distribution of food rations to half of Gaza's 1.5 million population.

"We have no food," said UNRWA spokesman Chris Gunness. "Our warehouses are empty."

Israel also cut off European Union-funded fuel supplies to Gaza's sole power plant on Thursday, prompting it to shut down.

Humanitarian agencies warned that the closure would lead to a further deterioration of the already precarious situation.

"It means children, mothers, elderly people, among the most vulnerable and disadvantaged in the Middle East, are not going to get UN assistance," Gunness told AFP.

The European Union said international law requires the provision of essential services to civilians.

"I call on Israel to reopen the crossings for humanitarian and commercial flows, in particular food and medicines," said EU Commissioner for External Relations Benita Ferrero-Waldner.

"Facilitation of fuel deliveries for the Gaza power plant should be resumed immediately," she said.

Israeli authorities initially said 30 truckloads of supplies would be allowed into Gaza on Thursday, but then turned the vehicles back.

Israel had been expected to ease its embargo after the truce, intended to last six months, went into effect in June, but Jerusalem argues that attacks by militants have made this impossible.

Israel has nevertheless indicated it would like to extend the ceasefire beyond its scheduled expiry on December 19, but suggested it is also prepared to get tough.

Hamas said it would continue to respond to what it called Israeli violations of the truce.

"Our rockets are a message saying we'll respond to any violation," one of its leaders, Khalil al-Hayya, told a rally of thousands in Gaza City.