Police and a top aide to radical Shi'ite cleric Muqtada al-Sadr reported Sunday that 18 people were killed in a clash involving US and Iraqi army forces at a mosque in eastern Baghdad. American military spokesmen said they were investigating the report. Abdul-Zahra al-Suaidi, head of al-Sadr's office in Baghdad, said US forces and Iraqi army soldiers opened fire at the al-Moustafa Shi'ite mosque in the Ur neighborhood, killing 18 people in what he called an unprovoked attack. Separately, Iraqi police Lt. Hassan Hmoud said 18 people were killed in the mosque. He said he had no other details.