They said armed men had fired rocket-propelled grenades at the intruding forces after a stand-off at the Thi al-Muraid mosque on Friday. There was no immediate word on casualties.
The mosque imam, Shaikh Rayan Tawfiq, said earlier that US and Iraqi forces had broken into the compound of the mosque in eastern Mosul to arrest unidentified suspects.
Worshippers attending Friday prayers resisted at first without weapons when the soldiers tried to enter the mosque, he said. But then US troops triggered an uproar when they entered the women's section of the mosque.
"I did my best to calm the people, but we don't want any Americans or any security organisation to go into the mosque under the pretext of arresting people," Tawfiq said before the firefight began.
In a separate development, Iraqi police fired bullets into the air to prevent supporters of Shia leader Muqtada al-Sadr from praying in Kufa.
US troops said they were looking
for suspects among worshippers
Policemen on Friday ringed the Suhaila mosque in Kufa, near the Shia Muslim learning centre of Najaf, and stopped about 150 al-Sadr followers from praying there, witnesses said.
Al-Sadr, a 32-year old leader who has defied the Shia religious establishment and led resistance against the US-backed government, used to pray in Kufa along with his followers before US forces drove them from the area in August.
Under an agreement with the government brokered by Ayat Allah Ali al-Sistani, Iraq's most influential Shia leader, al-Sadr agreed to pull out his fighters from Najaf and Kufa. The agreement effectively ended the resistance he was leading.
Al-Sadr, whose whereabouts are unknown, reopened his office in Najaf last week. A statement distributed by his office asked his supporters to return to pray in Kufa.