CNN quoted Iraqi officials as saying the insurgents had planned to seize control of the city and surrounding province and kill top Shia religious figures -- including Grand Ayatollah Ali al-Sistani, Iraq's most revered Shia leader.
A US helicopter was shot down during the fighting, which began Sunday morning and eased around 4 am Monday. The bodies of the two pilots were recovered. Iraqi sources said three Iraqi soldiers and two policemen were killed and 30 soldiers injured.
Latest reports said US and Iraqi forces had cordoned off the city, and were conducting mopping up operations on the outskirts.
The attack was planned to coincide with the ceremonies to mark the martyrdom of Imam Hussein, a grandson of the Prophet Mohammad, for which thousands of Shia pilgrims have arrived in Najaf and neighboring Karbala. The main ceremony culminates Tuesday with huge public processions.
Provincial Governor Assad Sultan Abu Kilel was quoted as saying the the insurgents were members of a previously unknown group called the Army of Heaven, comprising loyalists of executed former President Saddam Hussein.
Saddam's execution: What next?
Elsewhere in Iraq, the sectarian carnage continued. A series of bombings across Baghdad on Monday killed at least five people and wounded 15 others, an Iraqi Interior Ministry was quoted as saying.
At least 10 people including three children and four women were killed when mortars hit the Shia neighborhood in Jurf al-Sakhar, a small town nearly 50 km south of Baghdad Monday.
On Sunday, mortar shells hit a girls' school in a Sunni Arab neighborhood of Baghdad, killing five students and wounding 20.
