NABLUS, West Bank, April 9 (IslamOnline & News Agencies) – In defiance of U.S. and world calls for immediate withdrawal, Israeli F-16 warplanes were used Tuesday to bomb al-Yasmina district of Nablus' Old City, after the Israeli army failed to break the tough Palestinian resistance there.

The Israeli planes fired four missiles into the district where Palestinian resistance continued despite several days of heavy combat, witnesses told Agence France-Presse (AFP).

The F-16 strikes - the first since Israel launched its invasion of West Bank cities 12 days ago - came just hours after the Palestinian resistance killed 13 Israeli soldiers in bitter fighting in Jenin's refugee camp.

The use of the F-16s came also hours after the U.S. administration reiterated its calls on Israel to withdraw ‘without delay’ from the Palestinian areas.

In Washington, White House spokesman Ari Fleischer said President Bush "believes all parties still have responsibilities, [he is] still looking for results. 

"Israel's [responsibility] is withdrawal and to do so now. The Arab nations' responsibilities are to exercise statesmanship and create an environment for peace by condemning terrorism, by stopping the funding of terrorists," reported BBC’s online news service.

In London, meanwhile, Britain urged Israel to respect international law and said that a limited withdrawal of its forces from Palestinian territories "is not good enough".

"Like every other country, Israel has a right to security, but the Israeli government must respect international law, including international humanitarian law," Foreign Secretary Jack Straw said in a statement today.

"Israel's response to the demands of the international community for an early withdrawal from the Palestinian areas is not good enough," he added.

"The limited withdrawal Israel has announced falls short of the requirements of UN Security Council Resolutions 1402 and 1403."

Hawkish Israeli Prime Minister Ariel Sharon vowed that Israel would fight the Palestinians to the finish after Israeli soldiers were killed in the ferocious battle at Jenin refugee camp.

A spokesman for Britain's Foreign Office said: "It is not acceptable for the Israeli forces to stay in the Jenin refugee camp when the international community, including the (UN) Security Council, have called on Israel to pull out. "We will continue to press Israel to withdraw its forces."

"Today was a very hard day of fighting," Sharon said after a group of reservists were killed when a booby-trapped building exploded in the Jenin camp and the others were gunned down running to the rescue.

The army's heaviest daily loss in the 18-month-old Palestinian Intifada against the Israeli occupation jolted a military that takes pride in its prowess and minimal casualties. It also underscored the danger it faces from the almost armless Palestinians.