US marines have entered Saddam Hussein's hometown of Tikrit, American officials say.
Tikrit is Saddam Hussein's hometown and former stronghold
Coalition commanders in Qatar say their troops are meeting some resistance but it is very patchy.
Tikrit is believed to be a possible remaining stronghold of Saddam Hussein's regime and there has been speculation that troops loyal to the deposed leader might be planning a last stand there.
US marines are reported to be fighting Iraqi forces, including tanks, on the southern outskirts of Tikrit, about 140 kilometres (90 miles) north of Baghdad.
A Canadian journalist with the US forces told CNN: "It's a very, very significant attack. They've brought forward a great number of Cobra assault helicopters and there are Marine F-18s (aircraft) overhead."
A US military spokesperson in Qatar, said Task Force Tripoli, made up of members of the First Marine Expeditionary Force, was involved in fighting "in and around Tikrit".
About 250 US armoured vehicles have entered the town, and Brigadier John Kelly said five Iraqi tanks were destroyed on the outskirts and at least 15 Iraqi soldiers killed in firefights.
In Baghdad, many residents blame the US for not stopping the looting
In cities which have already fallen to coalition forces, the US military has promised to take steps to restore law and order after the collapse of the regime led to widespread looting and violence.
In Baghdad, hundreds of members of the Iraqi police force and public service workers have responded to an American call to help restore order.
About 1,000 people, including health workers, electricity and water ministry employees, attended a meeting in the centre of the capital to volunteer for work.
Early on Monday US marines engaged in a gun battle with snipers in central Baghdad. The exchange occurred outside the Palestine Hotel - where many international journalists are staying.
After the battle the marines took away at least one man.
Meanwhile, former Iraqi interior minister Watban Ibrahim al-Tikriti - a half-brother of Saddam Hussein - has been captured and flown by US helicopter to an unknown location for interrogation.
A Kurdish spokesman said he had been detained in northern Iraq, close to the Syrian border.
In other developments:
- Seven US soldiers who were being held by Iraqis are rescued by US forces travelling from Baghdad to Tikrit
- BBC correspondent in Kirkuk says returning Kurdish refugees have threatened the Arab families who now live there with eviction
- US President W Bush again warns Syria against harbouring former Iraqi leaders
- General Tommy Franks says coalition forces have DNA samples from Saddam Hussein and other Iraqi leaders and are using forensics to track them down
- An armed mob reportedly surrounds the Najaf house of a pro-Western Shia cleric, Ayatollah Mirza Ali Sistani, and is giving him until Monday to leave the country
The Republican Guard positions in Tikrit have been pounded by US planes in recent days.
Coalition commanders say most of the Republican Guard posted to defend the town has already been destroyed by bombing raids or has simply fled.
But the commander of US-led forces in Iraq, General Tommy Franks, warned the war would not end until all pockets of resistance were overcome.
He said there were still militia loyal to Saddam Hussein who were willing "to fight to their last breath".
Earlier on Sunday, US television network CNN showed live pictures from a team that drove into a military base in Tikrit, and found abandoned tanks. But the team later retreated after coming under fire at a checkpoint.
A Western journalist who has visited Tikrit say residents there told him they were prepared to surrender to advancing American forces.
The journalist, from the French news agency AFP said a tribal leader in Tikrit told him to convey a request to the US to stop bombing the city and allow local leaders 48 hours to negotiate the surrender of Fedayeen fighters still loyal to Saddam Hussein.