The arrest occurred a day after a U.S. military spokesman said that, in the past week, Iraqi and U.S. forces had captured 212 weapons caches around the country, with growing evidence of an Iranian link.

"This is a significant capture of a top special groups leader," said Navy Capt. Vic Beck, a military spokesman. "Special groups" is a term the U.S. uses to describe Iranian-backed Shiite Muslim militias it says have broken with anti-U.S. cleric Muqtada al-Sadr and refused to follow his cease-fire order.

The military said in a statement that intelligence reports led troops to the suspect, who was not identified, and he and another suspect were arrested without incident.

A roadside bombing in Mosul, meanwhile, killed three civilians and injured four others, according to a police officer who spoke on condition of anonymity because he wasn't authorized to release information. The city 360 kilometers (225 miles) north of Baghdad is what the U.S. describes as the last major urban stronghold of al-Qaida in Iraq.

Back in Baghdad, the main suspect arrested was reportedly in charge of all Shiite militia fighters in the western half of the city. The area west of the Tigris River that divides the capital has been a Sunni stronghold but has seen an increased Shiite presence with sectarian cleansing that peaked after the Feb. 22, 2006, bombing of a Shiite mosque in Samarra.

The military said the suspect was responsible for providing weapons to militia members, including armor-piercing bombs known as explosively formed penetrators, or EFPs, which U.S. officials say come from Iran. Tehran denies the allegations.

The man also allegedly selected fighters for paramilitary training and was an associate of other senior criminal leaders involved in attacks on U.S. and Iraqi security forces, the military said.

Al-Sadr pledged last year that his Mahdi Army last year would abide by a six-month cease-fire that has helped substantially decrease violence in Iraq, and that expires at the end of this month. But rogue groups have spun away from his organization and are receiving both Iranian training and weapons, including the EFPs, the U.S. says.

Rear Adm. Gregory Smith, a U.S. spokesman, said Feb. 17 that the military has seen an increase in the use of weapons by Iranian-backed Shiite extremists. Many of the caches had been in Iraq for some time and Smith declined to link them to an increased flow of weapons into the country, but he said training and financing of the groups continues.

"In just the past week, Iraqi and coalition forces captured 212 weapons caches across Iraq, two of those coming from here inside Baghdad, with growing links to the Iranian-backed special groups," he said at a news conference.

"The Iranian-backed special groups in particular are volatile in the sense that they receive specific training inside of Iran," Smith added. "What we don't know precisely is whether or not there's any direction coming from Iran as to how they conduct their operations inside of Iraq. We do think that the training and financing of those activities remain in place."

Prime Minister Nouri al-Maliki spent the weekend touting the successes of the security operation that began one year ago and peaked last summer with the influx of thousands of U.S. troops. The operation helped restore some security to a country that in January 2007 was on the brink of civil war.

Smith said insurgent attacks had declined by 60 percent over the past year, but cautioned the war was not yet won.

While officials celebrated security gains, Baghdad was hit by violence again Feb. 17 when a female suicide bomber struck a Shiite neighborhood, detonating after soldiers fired three bullets at her. The U.S. military said Feb. 19 such attacks demonstrate the desperation of al-Qaida - female bombers have been used only occasionally since the war began, though two women were the attackers in a pair of explosions at pet markets this month that killed almost 100 people.

Casualty figures were disputed in the Feb. 17 attack. Two doctors and a police officer, who spoke on condition of anonymity because they weren't authorized to release the information, said four people were killed and 12 wounded. The U.S. military, however, said the only death in the explosion was the bomber, although two Iraqi army soldiers were wounded.

According to the military, the bomber appeared to be a beggar with something bulky around her waist. When Iraqi troops asked her to raise her hands, she only lifted one and wires were seen in her other hand. After being shot at, she staggered to a nearby shop and blew up. It was unclear what triggered the blast.

Female suicide bombers have been involved in at least 18 attacks or attempted attacks since the war began, including the grisly bombings of two pet markets that killed nearly 100 people on Feb. 1. Iraqi and U.S. officials have said the women used in the pet market attacks were mentally disabled and apparently unwitting bombers.

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