BAGHDAD, Iraq — A protracted firefight between Marines and insurgents in a Fallujah suburb on Monday culminated with U.S. helicopter gunships and tanks firing at a mosque and toppling the minaret, further dimming hopes for a peaceful end to the three-week siege.

The U.S. command said that the battle erupted when insurgents, breaching a shaky cease-fire in Fallujah, 30 miles west of Baghdad, used the mosque to launch rocket-propelled grenades and small-arms fire at Marine positions. After two hours, pinned down by fire, the Marines called in helicopters and tanks, which directed "suppressing fire" at the mosque, the command said.

One U.S. Marine was killed and eight others wounded in the battle, which also killed eight insurgents, an American spokesman said. He said commanders still intended to go ahead with a plan to send U.S. troops on joint patrols with Iraqi security forces into contested parts of the city. But that plan, put forward by Fallujah civic leaders on Sunday to avert an American invasion of the city, appeared to be in jeopardy.

With Iraq's prospects of resuming progress toward a peaceful handover of sovereignty on June 30 hanging uneasily in the balance, developments in Fallujah were echoed by fresh tensions at Najaf, 100 miles south of Baghdad, that has been the focal point of a separate confrontation.

At nightfall on Monday, Najaf residents said a major battle was being fought on a key highway leading to the city by U.S. troops and militiamen loyal to Moktada al-Sadr, the rebel Shiite cleric who has holed up in Najaf and adjacent Kufa.

Sketchy reports of the Najaf battle suggested that al-Sadr's fighters had taken heavy casualties from U.S. ground troops and helicopter gunships.

In another development the Americans were watching, reports from inside Najaf said that the growing anger of residents there against al-Sadr and his men, who have sown a pattern of lawlessness since their uprising in the city began earlier this month, had taken a startling new turn, with a shadowy group killing at least five militiamen on Sunday and Monday.

These reports, from residents who reached relatives in Baghdad by telephone, said that the killers called themselves the Thulfiqar Army, after a two-bladed sword that Shiite tradition says was used by the patron saint of Shiism, Imam Ali, the martyred son-in-law of the Prophet Mohammed. The group distributed leaflets in Najaf threatening to kill members of al-Sadr's Mahdi Army, unless they fled Najaf immediately, according to accounts.

One Najaf resident said that some of al-Sadr's militiamen were shedding the black clothing that has been their signature. The same resident said he knew of two killings of Mahdi Army members on Sunday and that three others had been killed later on Sunday or Monday.