Hamas Islamists fired long-range rockets at a southern Israel city on Friday after an Israeli air strike on their Gaza stronghold in the 11th day of skirmishes that threaten a five-month-old truce.

The armed wing of the Islamist group said it fired five Grad rockets, the longest-range weapon it has used against the Jewish state. Israel said they hit Ashkelon, north of Gaza on the Mediterranean coast, with no casualties.

Israel and Hamas blamed each other for the flare-up since November 4, in which 12 Hamas militants have been killed by Israeli forces and scores of rockets fired into Israel. But both shied away from declaring an end to the Egyptian-brokered truce.

"We will continue to forcefully defend Israeli soldiers and citizens, to thwart attempts to stage attacks when we discover them," Israeli Defence Minister Ehud Barak said. "At the same time, if the other side wants to continue the calm we will definitely give it positive consideration."

Hamas took a similar stand.

"Up to this moment we are committed to the ceasefire," said Mahmoud al-Zahar, a Hamas leader. "Self-defense and resistance" would continue. "We are waiting for the Israelis. If they are really committed (to a truce) we have to address that frankly."

The 1960s-era Soviet-made Grad rocket has a range of 25 km (15 miles). Two of them struck Ashkelon.

Earlier, Palestinian medics said two Hamas fighters were wounded in an Israeli air strike, which a military spokesman said was in response to an earlier rocket attack.

After the air strike, Hamas said it fired eight shorter-range Qassam rockets aimed at the city of Sderot.

Two Qassams hit, causing damage to buildings, an Israeli police spokesman said. One Israeli was treated for shrapnel wounds and a number of people suffered shock.

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Israel's caretaker prime minister, Ehud Olmert, said in a statement after consultations with defense chiefs that Israel would not tolerate the rocket fire. It would continue to apply economic pressure on Hamas through border crossings.

Israel has not allowed humanitarian supplies into the Gaza Strip since November 4, when its troops raided the territory to destroy what the army described as a tunnel built by militants to kidnap Israeli soldiers.

Six Hamas gunmen were killed in the raid. Militants responded to the incursion with rocket salvoes.

Israel said the crossings would remain shut for now.

United Nations Secretary-General Ban Ki-moon called on Israel to allow urgently fuel and humanitarian aid into Gaza, where 750,000 Palestinians are in need of food.

Short of fuel, Gaza shut down its sole power plant, and rationed electricity it gets from Israel and Egypt. Some Gaza bakeries posted notices on Friday limiting the purchase of bread, although no major shortages were reported.

The EU also urged Israel to let aid supplies through.

"I am profoundly concerned about the consequences for the Gazan population of the complete closure of all Gaza crossings for deliveries of fuel and basic humanitarian assistance," External Relations Commissioner Benita Ferrero-Waldner said.

Israeli troops also killed four gunmen in a raid on Wednesday, prompting more rocket and mortar attacks from Gaza.

Hamas is in conflict with the Fatah faction of Palestinian President Mahmoud Abbas which holds sway in the Israeli-occupied West Bank and is negotiating with Israel on peace terms.

The rift between them widened in 2007 as Hamas took control of Gaza. Egypt brokered the Israel-Hamas truce, but Palestinian unity talks it is mediating faltered earlier this month.