BAGHDAD, Iraq – The governor of Baghdad severed all local government relations with the American military yesterday as outrage spread over claims that American troops attacked a Shiite mosque, killing 20 worshippers and the 80-year-old imam.

The bodies were laid out in a community hall in east Baghdad and throughout the day mourners shouted abuse at America, despite American insistence that its troops were not responsible.

Further condemnation was expressed by a spokesman for the prime minister, Ibrahim al-Jaafari, and the populist Shiite cleric Moqtada al-Sadr, whose supporters predominate in the mosque area.

Negotiations on the new government were suspended for the day as Shiite politicians issued a statement attacking the Americans for perpetrating such a “cowardly” act.

The breakdown in American-Shiite relations occurred as Iraq suffered another bout of bloodshed. At least 40 people were killed by a suicide bomb near the northwestern town of Tal Afar and 21 bodies were found, many with nooses around their necks. They were believed to have been the victims of sectarian violence.

The suicide bomber struck at an army recruitment centre. In a speech only last week President Bush cited Tal Afar as a counter-terrorism success story since it was recaptured from rebels last November.

Widely differing accounts were given of what happened at the Mostafa mosque, where the 20 Shiite were said to have been killed on Sunday.

Mr. al-Sadr’s Baghdad representative said that American troops tied up and executed worshippers. The police said that a gun battle had broken out between American forces and members of Mr. al-Sadr’s militia.

The Americans insisted that the incident did not occur at all.

They said that Iraqi special forces attacked “a kidnap cell” nearby, killing 16 insurgents and saving an Iraqi hostage, but that no American troops were in the area and no mosques were entered.

But in Iraq perception is as important as reality and throughout the day state television broadcast footage of a tangle of male bodies with gunshot wounds lying on what the presenter said was the floor of the imam’s living quarters.

On Saturday, the American ambassador, Zalmay Khalilzad, issued a warning to Shiite militias, saying that their activities must be curbed, as they had killed more Iraqis than “the terrorists” since the destruction of the Golden Mosque in Samarra. Both Mr. al-Sadr, who is supported by Baghdad’s governor, and the Supreme Council for Islamic Revolution in Iraq, the largest Shiite party and the dominant party in the present government, have militias.

Both have also recently expressed concern that the Americans, whose invasion and political process brought them to power, were now engaged in “the second betrayal.”