UN Secretary-General Kofi Annan has warned the United States, Britain and Iraq that an assault on Falluja could further anger Iraqis and undermine planned January elections. Annan's warning came in letters to US President George Bush, British Prime Minister Tony Blair and Iraqi Interim Prime Minister Iyad Allawi sent late last week, UN officials said on Friday, speaking on condition of anonymity. A UN spokesman said the world body would have no comment on the letters. "We have nothing to say about private communications between the secretary-general and heads of state," the spokesman said. But speaking in Brussels on Friday, Allawi said time was running out for a peaceful solution to end the resistance in Falluja. **Roads blocked*** "The window is closing for a peaceful settlement," he told a news conference at a European Union summit in the Belgian capital as US troops surrounded the town, west of Baghdad. Witnesses said the US military blocked roads round Falluja on Friday and mounted air strikes on parts of the city ahead of a major offensive to crush the fighters. US troops urged civilians to flee Falluja ahead of the offensive. US forces sealed all roads to the town and used loudspeakers and leaflets to tell residents they would detain any man under 45 trying to enter or leave the city. **Orders awaited*** Marine Colonel Michael Shupp told Reuters near Falluja that final preparations were on and that it would be over soon. "We are just awaiting orders from Prime Minister Allawi," he said. Many residents have already fled the town with a population of 300,000, about 50 km west of Baghdad. US warplanes and artillery bombarded Falluja on Friday, residents said, and fighters fired mortars and rocket-propelled grenades at US forces on the city outskirts. There was no immediate word on casualties. Ambulances were waiting for fighting to ease before venturing on to the streets. The US military said air raids on Thursday night had destroyed a command post, arms caches and resistance positions. Hospital workers in Falluja said the strikes had killed three people, wounded four and destroyed five houses. The military also said two marines were killed and four wounded in combat on Thursday in Anbar province, home to Falluja and the city of Ramadi, also expected to be the target of US assault. **PHOTO CAPTION*** A Shiite soldier of the New Iraqi Army shows a picture of Ayatollah Ali Sistani, after a training session at a base near the city of Falluja in western Iraq, November 5, 2004. (REUTERS)