BAGHDAD - US troop reinforcements have begun arriving in Baghdad to help Iraqi forces try to regain control of the streets amid worsening sectarian violence that US generals fear is pushing the country towards civil war.

Meanwhile, in the northern town of Tikrit, a suicide bomber blew himself up at a mourning ceremony, killing at least 10 people and wounding 20, police said.

Iraq's Ministry of Defence on Sunday confirmed the arrival of the first units of the 172nd Stryker Combat Team in Baghdad from the northern city of Mosul after US soldiers were seen patrolling some of the capital's predominantly Sunni districts.

The units are equipped with the US military's newest armoured combat vehicle, the eight-wheeled Stryker, which military officials say is better suited to the urban terrain and will provide troops with more manoeuvrability and firepower.

US Defence Secretary Donald Rumsfeld last month extended the year-long tour of 3700 troops from the 172nd by four months and redeployed them to Baghdad to help curb communal bloodshed there that continues to kill scores every week.

The decision was seen as an admission that the seven-week-old Operation Forward Together, a security crackdown in the capital driven by 50,000 mostly US-trained Iraqi forces, had failed to quell the daily car bombings, shootings and kidnappings.

On Sunday night local time, three Multi-National Division soldiers were killed by a roadside bomb southwest of Baghdad, the US military said in a statement. There was no immediate word on the nationalities of the soldiers or details on the attack.

Police also said on Sunday they had found 12 bodies around the capital, while gunmen also shot dead a policeman on Saturday. Eleven of the bodies had been tortured; all had been shot in the head, typical features of sectarian killings.

In Ishaqi, 100km north of Baghdad, gunmen in three cars attacked two trucks carrying razor wire heading to a nearby US base, killing four Iraqis and setting the vehicles ablaze, police said.

The commander of the US Central Command, General John Abizaid, said last week that "sectarian violence probably is as bad as I've seen it, in Baghdad in particular". At least 100 people die in Iraq every day, according to UN figures.

The US military commander in Iraq, General George Casey, said in July that Shi'ite death squads were fuelling a cycle of violence, retaliating against Sunni al Qaeda militants fighting to overthrow the Shi'ite-dominated government.

Shi'ite militias, many linked to parties in Shi'ite Prime Minister Nuri al-Maliki's national unity government have grown increasingly powerful in recent months and analysts say they now pose a huge threat to the stability of Maliki's government.

A senior coalition official said last week that many Iraqi soldiers "don't feel they have the backing of their government in confronting the militias".

The US troop reinforcements would "give the government a stable platform to move ahead. Baghdad is consuming their attention", he added.

US military spokesman Lieutenant Colonel Barry Johnson would not confirm the arrival of the troops, saying the military did not discuss troop movements until they were completed.

But he described the 172nd Combat Team as "very experienced" and well-equipped for their task to restore order. "The Stryker also gives them an additional element - it is easily manoeuvrable in the urban environment," he said.

The lightly armoured Stryker can carry a nine-man infantry squad and is the first new military vehicle to enter service into the United States army since the Abrams tank in the 1980s.

Nebraska senator Chuck Hagel, a frequent Republican critic of President George W Bush's Iraq policy, said on Sunday it was irresponsible and wrong to send more US troops to Baghdad.

"Where we go from here ... is a cold, hard assessment that Iraq is not going to turn out the way we were promised it would, and that's a fact, not because I say it -- that's the way it's going," he said on US television.