UN chief Kofi Annan has warned Iraqi and coalition governments against an assault on the Iraqi city of Falluja.
Kofi Annan has been a critic of the war in Iraq
In a letter to the leaders of the US, UK and Iraq, Mr Annan warned that force risked alienating Iraqis when their support for elections was vital.
Iraqi leader Iyad Allawi called the letter "confused" and said time for a peace deal in Falluja was running out.
US forces are pounding suspected rebel areas on the city's outskirts, and a major attack is thought to be imminent.
The BBC's Paul Wood, with US marines outside the rebel-held city, said a substantial ground force left the US base just after sunset on Friday.
US-led troops had closed all roads in and out of the city, which they believe is housing several thousand insurgents.
Hours later, in the dead of night, came flashes and sounds of gunfire on the horizon where Falluja begins, our correspondent described.
He said no-one would confirm whether this marks the start of the operation to retake Falluja and hand it back to the Iraqi authorities.
US military officials stress that the decision lies in the hands of the Iraqi prime minister.
So far, publicly at least, Mr Allawi appears not to have issued an order for the operation to begin.
But he earlier warned that the Falluja operation could not be delayed for long.
Apr 2003: US paratroopers shoot dead 13 demonstrators
Nov 2003-Jan 2004: attacks on three US helicopters kill 25
Feb 2004: 25 killed in attacks on Iraqi police
31 Mar 2004: four US contractors killed
Apr 2004: US seals off city
May 2004: Siege lifted
June 2004: Zarqawi loyalists targeted in US raids - continuing to date
Oct 2004: Iraqi PM threatens military action if Zarqawi is not handed over
"The window really is closing for a peaceful settlement," he said after meeting EU leaders in Brussels.
"We hope the casualties [in Falluja] will be minimal, but we have to protect the majority from the minority of terrorists who are inflicting damage on Iraq," he told BBC News.
He said if the United Nations secretary general thought he could prevent insurgents in Falluja from "inflicting damage and killing", he was welcome to try.
But he said of Mr Annan's letter: "We don't know what his intentions were, it's a very unclear message."
Mr Annan wrote of his "increasing concern at the prospect of an escalation in violence... I have in mind not only the risk of increased insurgent violence, but also reports of major military offensives in key localities such as Falluja".
He added that force could alienate Iraqis and "reinforce perceptions...of a continued
Mr Annan said the UN wanted to help, but: "We need a
conducive environment if elections are to produce a
A UN spokesman would not comment on the letter, reportedly dated 31 October.
But a US spokesman at the UN dismissed the intervention.
"This issue is for the government of Iraq and those who are willing to help the people of Iraq," said Richard Grenell.
"Spectator nations and international organisations should do more in Iraq and not in New York," he told the AFP agency.
The UN has warned that continued violence in Iraq could make it very difficult to hold elections in January, as planned.
Mr Annan's letter again emphasises the division between his position and the US-led coalition over Iraq.
It is less than two months since the secretary general caused a storm by saying that the invasion of Iraq was illegal.