Israel's overnight bombing of highway bridges in northern Lebanon and strikes at a Hizbollah stronghold in south Beirut paralysed aid convoys on Friday and relief workers warned of a worsening humanitarian crisis. Israeli jets destroyed four bridges on the main coastal highway linking Beirut to Syria, stalling an eight-truck convoy carrying food, shelter material and other aid to the estimated 900,000 Lebanese displaced by the three-week-old war. Israeli jets targeted a bridge just north of Beirut at Maameltain and two further north at Madfoun and Halat. The bridge at Maameltein, just north of Beirut, was split along its centre by a huge crater which partially engulfed the crushed shell of a minivan. Further north another lay stretched out in the valley it once spanned. "This is the main, if not the only life line from outside Lebanon into this country to bring food, medicine, shelter and all the basic needs you can think of for the hundreds of thousands of people that fled their south because of the Israeli shelling or because of the fighting between all the factions, also the tens of thousands of people who are either still besieged in the south, unwilling to leave or unable to leave their villages. This was the only road, safe road for us and the only equipped road, so to speak, so trucks can move in from Syria and come down to Beirut and then to the people who are displaced or to the people still is the south," said U.N. spokesman Khaled Mansour. The UNHCR was forced to postpone trips around Beirut to assess the needs and deliver aid to up to 400,000 people living with host families or in schools and parks in the area. The U.N.'s World Food Programme (WFP) also called off planned convoys southwards to the port city of Tyre and Rashidiyeh after bombing in a southern Beirut suburb prevented drivers from reaching the convoys' launch point. A third planned convoy carrying food, water and sanitary supplies to the city of Jazzine departed as planned but humanitarian workers said lack of access to hoards of refugees was deepening the humanitarian crisis. On Friday, a Green Peace ship carrying 38 tonnes of medicine, baby food and milk arrived in Beirut. A ship for the Red Cross delivered five jeeps and more food and medicine supplies. A spokesman for the Red Cross said that the aid will be distributed in Beirut and the south, but expressed frustration at not being able to reach the isolated villages worse hit in the fighting. "We will start distributing to displaced people here in Beirut in the Chouf mountains and in the south. We also would like to reach the people who have stayed behind in the villages in the south where the bombardment and where the fighting is fiercest. It's very frustrating because we think that actually these people need these supplies the most," said Bart Rijs. U.N. agencies also warned a looming fuel shortage could bring electrical power plants, hospitals and water pumping stations to a halt in the coming days, threatening already poor sanitation among thousands living in crowded conditions and raising the risk of epidemics. Israel has given the go-ahead for the two tankers to pass through its naval blockade, but the owners of the ships feel the Jewish state's "security concurrence" is not strong enough a safety guarantee.