US officials have said they are not satisfied with progress made by Iraqi forces in Falluja, warning their forces may return to the city.

Brigadier-General Mark Kimmitt said on Saturday that while violence had abated in the city - the scene of heavy fighting until US-led occupation troops withdrew on 30 April - he was disappointed that the murderers of four American contractors had not been brought to book.

The US general refused to rule out a return of US troops, who were in April engaged in some of fiercest exchanges since the fall of Baghdad in April 2003.

"We retain the right to use any military option which is proportional within the rules of engagement to solve the problem in Falluja," he said.

Some 48 US occupation soldiers were killed in and around Falluja last April. More than 800 Iraqis - mostly civilians - were killed in intense bombardment of the city.

Not satisfied

"We're not satisfied that we're making adequate progress in the latter," Kimmitt said, referring to the "brutal murder and dismemberment" on 31 March of the security contractors employed by the Blackwater firm that protects US overseer Paul Bremer.

Falluja has seen some of the fiercest post-invasion fighting

The US also wants Iraqi forces to prosecute the perpetrators of a 14 February attack on Iraqi security force bases in the city, which resulted in the escape of at least 70 prisoners.

Kimmitt also demanded that the town's security force, known as the Falluja Brigade, stop "foreign fighters" from smuggling weapons.

No progress

The comments are the US military's strongest warning yet to the Iraqi forces patrolling Falluja, many of them veterans of Saddam Hussein's armed forces.

Kimmitt stressed that military intervention would be the US military's last option.

"We're carefully watching Falluja, we're certainly intending to try to accelerate the process for peaceful discussions rather than military discussions."