US forces yesterday warned residents of Fallujah through loud-speakers and leaflets that they would detain any man under 45 trying to enter or leave the rebel-held Iraqi city.
US troops also urged residents, in Arabic, to help them capture "terrorists" and warned women and children to leave the Sunni Muslim city, locals said.
The US military had no immediate comment.
Witnesses said US troops blocked roads around Fallujah and clashed with insurgents on the eastern and southeastern edges of the city yesterday. The US military also mounted fresh air strikes on parts of the city yesterday, wit-nesses said, ahead of a major offensive to crush insurgents.
They said insurgents fired rocket-propelled grenades and mortars at US troops who were shelling from around the eastern and southeastern edges of the city.
Hospital officials said they were waiting for clashes to ease before sending out ambulances. Overnight air strikes had killed three people and wounded four, they said. There was no immediate word from the US military on the latest clashes.
It said earlier that its warplanes had mounted five strikes on rebel targets in Fallujah within seven hours overnight, hitting an arms cache, a rebel command post and other targets.
Residents said five houses were destroyed.
They said troops had sealed off all roads in and out of the Sunni Muslim city of 300,000 people, including the highway leading to the borders with Jordan and Syria, as US Marines prepared for a full assault designed to crush the rebels before nationwide elections in January.
Syrian officials also said the Iraqis closed a crossing point on the two countries' common border.
US Marines say they are awaiting orders from US President George W. Bush and Iraqi Prime Minister Iyad Allawi, who is visiting Europe, before storming Fallujah and Ramadi, another Sunni city and rebel stronghold west of Baghdad.
Most of Fallujah's population has already fled.
Insurgents also killed two US Marines and wounded four others in fighting in the area, the US military said yesterday.
The two Marines were killed on Thursday in Anbar province, which includes Fallujah, but the Marines refused to say where and how they died. In addition, a US soldier was killed and another wounded when a roadside bomb hit their vehicle on Thursday north of Baghdad.
Three British soldiers were killed on Thursday south of Baghdad and eight others were wounded when a suicide driver blew up his vehicle at a checkpoint. An Iraqi translator also died in the attack -- the single biggest loss of life for the British since August last year.
The Britons were part of the Black Watch regiment, a Scottish unit shifted to central Iraq from the relatively quiet south to free up US forces for the planned assault on Fallujah and other insurgent bastions north and west of the capital.
British Prime Minister Tony Blair had agreed to a US request to move British troops to central Iraq despite considerable opposition at home, even within his Labour Party.
Scottish Nationalist Party spokesman Angus Robertson warned that the deaths would have "profound implications" for public opinion in Scotland, where the Black Watch regiment is recruited.
The deteriorating security situation prompted the humanitarian organization Medecins Sans Frontieres to announce it was closing its operations in Iraq. CARE International withdrew from the country after its national director, Margaret Hassan, was kidnapped last month.