US ground troops are pounding suspected rebel positions on the outskirts of the Iraqi city of Falluja amid expectations that a major assault is imminent.
US marines have been taking part in intensive drills
The BBC's Paul Wood, with US marines outside the rebel-held city, says a substantial ground force left the US base just after sunset.
US-led troops have closed all roads in and out of the city, and await the go-ahead from the Iraqi prime minister.
But the UN secretary-general has warned an assault could further anger Iraqis.
Our correspondent, who is working under US military censorship, said a column of Humvee jeeps and light armoured vehicles had set off from the base.
A US officer confirmed that marines were carrying out attacks in the outskirts of the city, but he did not elaborate.
The US military said it had carried out five air strikes on Friday, destroying a command post, arms caches and rebel positions.
US and Iraqi officials are aiming to flush out insurgents ahead of elections in January.
In Brussels, Iraq's interim Prime Minister Iyad Allawi said the window was closing to avert an all-out assault on the city, 50km (30 miles) west of Baghdad.
US-led troops called off a three-week siege of Falluja in April amid large numbers of civilian deaths.
The US military has been warning residents through loudspeakers and leaflets to leave the city. It estimates that fewer than 60,000 remain out of a population of 300,000.
UN Secretary General Kofi Annan has warned Iraq, the US and UK that a major assault could alienate ordinary Iraqis and disrupt the planned elections,
The BBC's Susannah Price at the UN says US officials were outraged by Mr Annan's letter, which highlights the divisions over the way to achieve peace in Iraq.
Mr Allawi said the letter, which he received a few days ago, was "very unclear". He said Mr Annan did not present a plan as to what to do, nor was it clear what the UN could do to stop insurgents attacking.
"We intend to liberate the people and bring the rule of law," Mr Allawi told a news conference at a European Union summit.
Earlier on Friday, US soldier was killed and five wounded when insurgents fired at their base on the outskirts of Falluja, the US military said.
Our correspondent says there is an assumption among the US marines that a major assault is inevitable and imminent. Units are stocking up with large quantities of artillery shells, and the pace of training has increased.
Mr Allawi has repeatedly threatened an all-out assault on Falluja if residents do not turn in Jordanian militant Abu Musab al-Zarqawi, who is thought to be based there.
Zarqawi's al-Qaeda-linked group has captured and killed hostages and is blamed for a string of suicide bombings.
US and Iraqi officials say several thousand fighters are holed up in the city.
The Iraqi authorities say the fighters are highly organised and led by former army officers from Saddam Hussein's army.
The BBC's defence correspondent Paul Welsh says the attack has been advertised for weeks - to try to persuade the militants to give up and the civilians to leave to escape danger.
In other developments:
Iraqi Vice-President Ibrahim Jaafari says the election date has been set for 27 January. Mr Allawi says no date has been set, although elections will take place by the end of January
Two children are killed in Muqdadiya, north-east of Baghdad, when a mortar shell intended for a nearby police station hits their home
Three hostages, two from Lebanon and one from Nepal, are released.