Fighting in besieged Iraqi city

US troops have halted the return of families to Falluja

US marines supported by tanks and aircraft have fought Iraqi insurgents in the besieged city of Falluja.

Wednesday's fighting reportedly began just after daybreak, when dozens of insurgents attacked US marines encircling the city.

The Americans said 20 Iraqi fighters were killed and three US marines wounded in the battle.

Residents of the city said six civilians were killed and 10 wounded by US fire.

US officials say the guerrillas are jeopardising a peace agreement reached on Monday by not turning in their weapons.

In response, US forces have blocked Falluja residents who fled earlier fighting from returning to the predominantly Sunni Muslim city.

'Tit-for-tat'

The surrender of heavy weapons by Falluja's insurgents is a key demand of the deal, and US officials insist that the agreement's success hinges on whether the Iraqi negotiators - a group of local civic leaders - can convince the insurgents to comply.

FALLUJA Population: 284,500 (2004 estimate, World Gazetteer) Predominantly Sunni Muslim with an estimated 70 mosques Benefited from heavy investment in factories under Saddam Hussein In pictures: Families wait US defends Falluja tactics

Allowing refugees to return is a key American commitment under the deal.

About 10 families made it back into the city on Wednesday morning before US marines announced that no more would be allowed to enter.

Marine Lt Col Brennan Byrne said: "It's tit-for-tat, we're not seeing tat."

He added that high-level commanders viewed the insurgent attack - in which an assault with rocket-propelled grenades and small arms fire was countered by marines backed by helicopter gunships and F-16 fighter-bomber aircraft - as a "major breach" of the agreement.

Tens of thousands of Falluja's residents fled the city after fighting broke out on 5 April.

Tuesday - the day the agreement began being implemented - saw 50 families allowed back into the city by US forces.

'Remote' chance of peace

US commanders have warned that they could launch an all-out attack on the city if the agreement falls through.

US Defence Secretary Donald Rumsfeld has said the chances of a negotiated solution to the stand-off are "remote" and has warned that the US military will not stand by indefinitely.

Since the Americans laid siege to Falluja more than two weeks ago, hospital sources say the fighting has killed about 600 Iraqis.

Scores of US marines have also been killed in the bloodiest fighting since the toppling of Saddam Hussein a year ago.

The marines launched their offensive after four US security contractors were killed in the city, and the bodies of two of them were mutilated.