BAGHDAD -- A suicide car bomber killed one US soldier and wounded 18 in northern Iraq on Sunday, as 13 civilians died in separate strikes in the heart of Baghdad and elsewhere in the country, officials said.

The attacker blew up his explosives-laden vehicle near a small patrol base just north of executed president Saddam Hussein's hometown of Tikrit, the US military said.

It put the toll at one soldier killed and 18 plus two Iraqi contractors wounded.

A roadside bomb at a police centre in the Al-Yarmuk district of west Baghdad, meanwhile, killed four civilians and wounded 23, interior ministry and hospital officials said.

The victims were waiting to enlist in the force when the bomb exploded. Iraq's security forces and police recruits have been a frequent target for insurgent attack in the violence-ravaged country.

On Saturday, a suicide bomber ploughed his vehicle into a police car in the same district, killing a policeman and a civilian and wounding six other people.

In other violence, a deadly mortar attack Sunday on the Green Zone, which houses the US embassy as well as Iraqi government offices, killed three people and wounded seven, an official said.

The interior ministry official said the mortar fire apparently targeted the defense ministry but that the round fell short striking one of the entrances to the compound.

Just over a week ago, a Filipino was killed and two female compatriots wounded in another mortar attack on the Green Zone.

Northeast of the capital, unidentified gunmen stormed a market in Qazaniya, killing five people in the town near the border with Iran, police said.

South of the Iraqi capital, in Iskandiriyah, a civilian was killed when a bomb targeting a police vehicle detonated, officials said.

The US military meanwhile reported that a US soldier was killed on Saturday when his vehicle was hit by a roadside bomb as it travelled through the east of the capital.

The latest deaths brought to 4,093 the number of American soldiers killed since the March 2003 invasion, according to an Agence France-Presse tally based on independent website www.icasualties.org.

Last month, the US military saw its lowest monthly death toll since the invasion. Just 19 troops were killed, undercutting the previous low of 20 recorded in February 2004.

US forces, meanwhile, said they had arrested the suspected commander of an assassination squad operating in the southern oil city of Basra believed to have links to Iran.

Troops swooped on a hideout in the Rusafa district of east Baghdad late on Saturday after receiving information from other captured fighters, arresting the man and a colleague, a statement said.

The man detained is also suspected of arms dealing and forging documents as well as moving militiamen in and out of Iran for training, the military said.

West of the capital, Iraqi police dismantled an Al-Qaeda cell of would-be suicide bombers in the town of Hit on Sunday and seized 50 explosive belts primed for use, mayor Sheikh Hikmat Jubair al-Kaud said.

Hit lies in Al-Anbar province, which was a bastion of the Sunni Arab insurgency that erupted shortly after the US-led invasion but has seen a sharp fall in violence since the US military began recruiting local tribal chiefs to fight Al-Qaeda 18 months ago.

According to the mayor, the dismantled cell carried out a June 1 attack on a police checkpoint in Hit that killed nine people, including an officer.

The US army said it made 49 arrests after that attack.