An Israeli air strike killed at least 40 Lebanese civilians, including 21 children, in the southern village of Qana on Sunday, in the bloodiest single attack during Israel's 19-day-old war on Hizbollah. The deadly air strike, whose target was not immediately clear, occurred as U.S. Secretary of State Condoleezza Rice was in Jerusalem on a mission to persuade Israel and Lebanon to agree on an international force to deploy on the border. The three-storey building where about 60 displaced civilians were sheltering and several other houses were destroyed in the dawn raid, catching many people asleep, a Reuters witness said. The witness counted 40 bodies, including those of 21 children. Rescue workers and witnesses said more bodies were still trapped under the rubble. Lebanese Red Cross workers covered the corpse of one dead child with a blanket. A woman in a red-patterned dress lay crumpled and lifeless in the broken masonry. A leg poked out from the rubble nearby. A child lay dead in the street. Israel's military said it had warned residents of Qana to leave and said Hizbollah bore responsibility for using it to fire rockets at the Jewish state. Distraught people in Qana screamed in grief and anger amid the rubble of wrecked buildings as others scrabbled at slabs of concrete with their bare hands to try to reach those still buried in the debris. At least 523 Lebanese, mostly civilians, and 51 Israelis have been killed in the conflict that erupted after Hizbollah captured two Israeli soldiers in a cross-border raid on July 12. In other violence, Hizbollah fighters battled Israeli soldiers making a new thrust into southern Lebanon. Sunday's clashes erupted when Israeli soldiers crossed the Israeli border towards the town of Khiam. Israeli aircraft and artillery pounded the Shi'ite town while guerrillas fired rockets at northern Israel, Lebanese security sources said. Convoys carrying aid for the United Nations World Food Programme (WFP), scheduled to leave to the south on Sunday, said they had to postpone their travel because they did not get security clearance. A spokesman said they did not get clearance from Israel's Defence Force. He said they are scheduled to deliver aid to the southern areas of Tyre and Tbinin on Monday. Since the crisis began almost three weeks ago and the continuos bombing of roads and bridges, movement on the ground has been difficult and dangerous. Aid workers say they are finding it impossible to get medical supplies and food safely to isolated villages in southern Lebanon due to the Israeli bombardment. On Friday, WFP sent two convoys to southern Lebanon carrying wheat, flour, meat and blankets for UN and other international and humanitarian organisations. A humanitarian corridor has allowed the United Nations to truck food and basic medical supplies to the southern port of Tyre, but getting safe passage beyond that is another matter. The United Nations estimates up to 800,000 Lebanese have been displaced since the bombing started. In Beirut, long queues of cars waited at petrol stations, following news that the country is suffering from fuel shortages. Petrol is still available in the capital Beirut, but it is running out. With Israel bombarding the main roads and bridges, supply routes to other parts of the country are either severed or hazardous. Israeli planes have attacked petrol stations and fuel storage tankers at the airport, power plants, and ports as well as trucks carrying various supplies. Southern Lebanon, which has seen the worst of Israel's bombardment, is facing shortages which are now spreading to the eastern Bekaa Valley, another region targeted by Israel. Diesel, used to feed power generators when the electricity supply is disrupted, is also in increasingly short supply, with fuel companies either rationing to customers, refusing to supply them, or refusing to deliver because roads are dangerous.