Homicide attackers wearing women's robes blew themselves up Friday in a Shiite mosque, killing 79 people and wounding more than 160, police said. It was the deadliest single attack in Iraq this year and the second major bombing of a Shiite target in as many days.

Police Lt. Col. Falah al-Mohammedawi said the blasts were caused by two homicide attackers wearing black abayas at the Buratha mosque, which is affiliated with the Supreme Council for the Islamic Revolution in Iraq, the main Shiite party.

Jalal Eddin al-Sagheer, the preacher at the mosque and one of the country's leading politicians, said there were three assailants. One came through the women's security checkpoint and blew up first, he said. Another raced into the mosque's courtyard while a third came to his office before detonating his bomb, said al-Sagheer, who was not injured.

He accused Sunni politicians and clerics of waging "a campaign of distortions and lies against the Buratha mosque, claiming that it includes Sunni prisoners and mass graves of Sunnis."

"Shiites are the ones who are targeted as part of this dirty sectarian war waged against them as the world watches silently," he told Al-Arabiya television.

The attack occurred as worshippers were leaving after Friday prayers, the main weekly religious service. Earlier Friday, the Interior Ministry cautioned people in Baghdad to avoid crowds near mosques and markets due to a car bomb threat.

Rescuers carried the bodies from the mosque compound on makeshift wooden wheelbarrows and loaded them on the backs of pickup trucks. The Baghdad city council urged Iraqis to donate blood for those wounded.

On Thursday, a car bomb exploded about 300 yards from the Imam Ali mosque in Najaf, the most sacred shrine in Iraq for Shiite Muslims. Ten people were killed, police said.

The attacks were likely to increase tensions between Sunni and Shiite Muslims, already at a high level following the Feb. 22 blast at a Shiite shrine in Samarra and reprisal killings. That bombing triggered a war of reprisal attacks against Sunni mosques and clerics.

U.S. Ambassador Zalmay Khalilzad condemned Friday's mosque attack, saying those who carried it out were the "enemies of all faiths and of all humanity." He urged Iraqis to refrain from retaliatory violence, and he separately warned that the country faces the possibility of sectarian civil war if efforts to build a national unity government do not succeed.

A senior Sunni politician also denounced the attack and called for unity among all Iraqis.

"Bloodshed is forbidden," Adnan al-Duleimi told Iraqiyah television. "I call on all religious figures and politicians to work together to avoid provocative acts of sedition."

The Interior Ministry, which oversees police, said it had received intelligence that insurgents were preparing to set off seven car bombs in Baghdad. Al-Mohammedawi said the alert would remain until the bombs were discovered and deactivated.

Security forces were searching the city, with orders to protect holy sites and be on the lookout for suspicious cars, the statement said, urging citizens to "be cautious, and to avoid gatherings or crowds while leaving markets, mosques and churches."

The statement also warned that legal measures would be taken against "any security official who fails to take the necessary procedures to foil any terrorist attack in his area." The Shiite-dominated ministry faces accusations of militia infiltration in its ranks.

Other car bombs were possibly heading to some southern Iraqi provinces as well, the statement said, putting security forces in the south on high alert.

The U.S. military also said Friday that three American troops were killed in separate attacks in Baghdad and north of the capital. The deaths raised to at least 2,349 the number of American forces who have died since the Iraq war started in March 2003, according to an Associated Press count.

One service member died Friday of wounds suffered while on patrol in western Baghdad, the U.S. military said. The statement said the victim's patrol had come under small arms fire.

Separately, the command said a soldier from the Army's 172nd Stryker Brigade Combat Team was killed Thursday when his combat patrol was struck a roadside bomb near Beiji, 155 miles north of Baghdad.

A Marine was killed in "enemy action" in Anbar province west of Baghdad, also Thursday, another statement said.

Khalilzad told the British Broadcasting Corp. that political contacts among Sunni, Shiite and Kurdish leaders were improving, but that within the general population, "polarization along sectarian lines" was intensifying — in part due to the role of armed militias.

He warned that "a sectarian war in Iraq" could draw in neighboring countries, "affecting the entire region."

"That's a possibility if we don't do everything we can to make this country work," Khalilzad said. "What's happening here has huge implications for the region and the world."

He said the best way to prevent such a conflict was to form a government including representatives of all groups. That effort has stalled over Sunni and Kurdish opposition to the Shiite candidate to lead the government, Prime Minister Ibrahim al-Jaafari.

Khalilzad avoided any criticism of al-Jaafari. He said many competent Iraqis were capable of leading the government and the current prime minister "certainly is one of them."

Khalilzad said the international community must do everything possible "to make this country work" because failure "would have the most serious consequences for the Iraqis, for sure, but also for the region and for the world."

Rising sectarian tensions — worsened by armed, religiously based militias and death squads — have emerged as a significant threat to U.S. efforts to form a stable society in Iraq.

Last month, Khalilzad said that "more Iraqis are dying today from the militia violence than from the terrorists," meaning Sunni-dominated insurgents.

In the BBC interview, Khalilzad cited the role of armed militias in sharpening sectarian tensions, including armed groups associated with Shiite political parties and Sunni insurgents.

"What I was saying to the Iraqis is that for the success of Iraq, this problem of unauthorized military formations have to be dealt with," he said, adding U.S. officials were working with the Iraqis to develop a plan for curbing militias and would insist that it be implemented.

Khalilzad also confirmed the Americans had been meeting with groups linked to the insurgency and said he believed those contacts were responsible for a decline in the number of attacks against U.S. and coalition forces. Last month, they suffered their lowest monthly death toll in Iraq since February 2005, although the casualty rate has increased somewhat in the first week of April.

He would not specify the groups but said they did not include Saddam Hussein loyalists or "terrorists," presumably excluding extremists of Al Qaeda in Iraq or the Ansar al-Sunnah Army.

U.S. officials have in the past confirmed contacts with people who claimed to have links with the insurgents. It was unclear whether these contacts included insurgent commanders or simply intermediaries who support the war against coalition forces.