An Iraqi army soldier kicks the door of a house during a search operation, 30 Jan 2007, two days after a raging battle in the town of Zarqa north of the holy city of Najaf

The U.S. military in Iraq says coalition forces have killed 18 insurgents during fierce fighting in the western city of Ramadi.

The military says the fighting which began late Thursday continued until Friday morning.

It said coalition forces carried out airstrkes after coming under fire from insurgents armed with rocket-propelled grenades and machine guns.

Ramadi is the capital of al-Anbar province, a stronghold of Sunni insurgents.

Separately, the military says it is investigating reports that an American helicopter has gone down today near the Taji airbase, about 18-kilometers from the capital.

Meanwhile, authorities in the southern city of Najaf have imposed a daytime ban on vehicle traffic, following fierce fighting this week between an Islamic cult and U.S. and Iraqi forces.

Officials said the overnight curfew has been extended and additional restrictions imposed in the Shi'ite holy city, about 160 kilometers south of Baghdad.

The officials did not say if there was any threat of an imminent attack on Najaf, but described the restrictions as preventive security measures.

On Sunday, more than 200 members of the "Soldiers of Heaven" cult, including its leader, were killed during fierce fighting that involved U.S. airstrikes. Hundreds of cult members were arrested.

Some information for this report was provided by AFP, AP and Reuters.