U.S. soldiers in Baghdad captured an Iraqi arms dealer and "assassination squad" leader responsible for trafficking Shiite extremists in and out of neighboring Iran for training, the military said Sunday.

The arrest came as Iraqi Prime Minister Nouri al-Maliki was visiting Iran and was expected to raise with its leaders the long-time U.S. allegations that Tehran arms, trains and funds Shiite Muslim militiamen inside Iraq. Tehran denies the charges.

The Iraqi prime minister, himself a Shiite, is struggling to keep Washington happy while reassuring Iran, the largest Shiite nation, that a proposed U.S.-Iraqi security agreement would not make his country an American launching pad for attacks on Iran.

The U.S. arrest campaign against Shiite militiamen with alleged ties to Iran was likely to be on the agenda for al-Maliki's talks with Iranian officials.

U.S. soldiers, acting on intelligence from other Shiite militiamen already in custody, captured the Basra-based "special groups" leader late Saturday at a hideout in eastern Baghdad, according to a military statement.

"The wanted man is alleged to be a commander of an assassination squad in Basra, an arms dealer with connections to Iran and a document counterfeiter," the statement said.

He also arranges transportation of criminals into Iran for training, and then back into Iraq, it said. One of the leader's aides was also arrested without incident.

The U.S. military uses the term "special groups" to describe Shiite fighters defying a cease-fire order from anti-American cleric Muqtada al-Sadr, whose militiamen fought American and Iraqi forces for seven weeks until a May truce.

Meanwhile, the military said in another statement that it captured six more suspected Sunni extremists Sunday in the northern city of Mosul, including an alleged al-Qaida in Iraq leader and another man who is a wiring expert in charge of a bombing cell there.

Two women were injured when American soldiers "breached the door of a target building" during the arrest raid, the statement said. Both were treated at the scene and then transported to an Iraqi hospital, it said.

Mosul is believed to be one of the last urban strongholds of al-Qaida in Iraq, and U.S. and Iraqi forces have battled with militants there in recent months.

Violence continued Sunday in Baghdad, where four new police recruits were killed in an attack on the National Police headquarters on the city's west side, where a blast went off near a gate where recruits were gathering, police said.

Another 22 people were wounded in the attack. Police gave conflicting reports about whether the attack used mortars or a roadside bomb.

A mortar shell landed just outside Baghdad's Green Zone midmorning, killing three employees and wounding seven others, police said. The mortar was apparently targeting the Defense Ministry, which is inside the U.S.-guarded diplomatic zone, but fell short, they said.

Mortar and rocket attacks were once a daily occurrence in the Green Zone in central Baghdad, but have fallen off in recent weeks.