At least 27 people were killed and scores more injured in a mortar attack on a mosque in the Iraqi city of Kufa. The attack came as thousands of followers of the revered Shia leader Grand Ayatollah Ali al-Sistani are obeying his call to march on the neighboring city of Najaf, where insurgents have been locked in a standoff with Iraqi and U.S. forces for the past three weeks.

Thousands of Muslims had gathered at the gold-domed mosque in the city of Kufa early Thursday when it was hit by mortar fire. Local television showed images of scores of dead and wounded, some lying in pools of blood in the mosque's courtyard. Witnesses say snipers also fired at the crowd after the mortar attack took place.

Most of the victims were believed to be supporters of radical Shia cleric Moqtada al-Sadr, whose followers have been engaged in three weeks of fighting with U.S. and Iraqi government forces in the nearby city of Najaf.

Aides to Mr. al-Sadr have blamed U.S. forces for the attack on the mosque - charges the officials have denied.

In Najaf, insurgents loyal to Mr. al-Sadr have been holed up in the Imam Ali shrine - a sacred site to the world's Shia Muslims. U.S. forces have conducted air strikes and artillery attacks on parts of Najaf's Old City throughout the standoff, but the shrine has been largely unaffected.

Another confrontation may be brewing. Thousands of supporters of Iraq's most respected Shia leader Grand Ayatollah Ali al-Sistani have clogged highways leading to the city of Najaf after the cleric called on them to march on the holy city.

Ayatollah al-Sistani has gone to Najaf as well, traveling in a convoy consisting of some 50 police and military vehicles.

The Ayatollah returned to Iraq Wednesday, after an absence of nearly three weeks. Aides say he had been receiving medical treatment for a heart condition in London.

Officials close to Mr. al-Sadr have also called on his followers to go to Najaf.