More than 10,000 US soldiers and Marines have taken positions around Fallujah for an expected assault on the city that has become the symbol of Iraqi resistance.
The rebel-held city has been under attack for a number of days and has come under renewed US air and artillery strikes tonight.
The US military build up comes as US Secretary General, Mr Kofi Annan, warned against an assault on the city.
Secretary General Annan warned the United States and Britain against an assault on Fallujah, saying it could undermine elections planned for January.
UN officials said Mr Annan warned the United States, Britain and Iraq against an assault on the rebel-held city in a letter delivered last weekend to US President George W. Bush, British Prime Minister Tony Blair and Iraqi Prime Minister Iyad Allawi.
A spokesman for the UN said the world body would have no comment on the letters.
Word of the warning surfaced as US forces warned residents to leave while also warning that they would detain any man under 45 years of age trying to enter or leave the rebel-held Iraqi city.
A spokesman for Iraq's UN mission said Ambassador Samir Sumaidy had requested an appointment with Mr Annan "to express the Iraq point of view on the letter."
But Iraq's first post-Saddam ambassador would not comment until after the meeting with the secretary-general, the spokesman added.
Annan 's warning reflects long-standing differences with Washington over how to best bring security to the country torn by an insurgency since the invasion launched in March 2003.
The US-led multinational force in Iraq is for now focused on retaking insurgent strongholds by force while UN officials have stressed that military might runs the risk of killing civilians and further alienating ordinary Iraqis.
In his letter, according to the Los Angeles Times, Annan acknowledged the need to restore security but said a broad political process that included groups not represented in the interim government would best lead to stability.