Annan threw his support behind UN envoy Lakhdar Brahimi, who said Tuesday that the ongoing bloodshed could have dramatic long-term consequences and was a threat to efforts to forge a new caretaker government in the coming weeks.

"The more the occupation is seen as taking steps that harm the civilians and the population, the greater the ranks of the resistance grows," Annan told a press conference here.

"Violent military action by an occupying power against the inhabitants of an occupied country will only make matters worse," he said.

With resistance mounting, US forces are locked in tense stand-offs in the cities of Najaf and Fallujah -- the first a holy site for Iraq's majority Shia Muslims and the second a hotbed of anti-occupation rage.

The US military said Wednesday it was still committed to a political settlement in Fallujah even as it pounded the city with a second day of air strikes as a weeks-long siege continued despite a tense ceasefire.

AFP obtained a copy of a letter to Annan from leaders in Fallujah asking for his immediate help.

"We are calling on you to personally (intervene) to stop the ongoing bloodbath," said the leaders, who claimed more than 800 people have been killed by US troops in the city.