Hamas has offered Israel a year-long ceasefire in the Gaza Strip if Israel pulls its forces out within a week and reopens border crossings immediately. Hamas and diplomatic sources said the truce would be renewable. Earlier, the UN headquarters in Gaza were set ablaze during some of the heaviest attacks in the 20-day-old Israeli offensive. The United Nations Relief and Work Agency (UNRWA) building was being used as a shelter by up to 700 civilians fleeing the bombardment of the Palestinian enclave when it was struck. Spokesman Chris Gunness said: "Our compound in Gaza has now received three hits, reportedly of white phosphorus. Buildings in the compound are on fire, there are loaded fuel tankers nearby. Three people have been injured. "It is not clear at this stage if they are UNWRA staff or some of the 700 or so civilians who have taken refuge in our compound."