Iraq's most senior Shia cleric has returned to the country after receiving medical treatment in London. He issued a call for his followers to march on the holy city of Najaf in a bid to end the fighting that has rocked the city for three weeks.

Grand Ayatollah Ali al-Sistani returned to Iraq in a massive convoy, escorted across the Kuwaiti border by Iraqi police and national guard units. He stopped in the southern city of Basra for the night before continuing on to Najaf on Thursday.

A spokesman for the cleric says Mr. al-Sistani has called on his followers to converge on Najaf to "stop the bloodshed" that has gone on for nearly three weeks.

Najaf is home to one of the holiest sites in Shia Islam, the Shrine of Imam Ali. Imam Ali was a cousin and son-in-law of the Prophet Mohammed.

Heavy fighting has raged around the shrine, and fighters from the Mahdi Army militia have been seen firing mortars and other weapons from inside the mosque complex. At one point, thousands of militants were holed up inside the shrine, but their numbers have recently dwindled to a few hundred determined fighters, at most.

As the clashes have continued, Iraq's defense minister has threatened to order his troops to storm the shrine. Iraqi officials, however, have been adamant that no U.S. forces will enter the holy site.

The man leading the Mahdi Army militia is radical Shia cleric Moqtada al-Sadr, a junior religious leader who has built his following largely on two things, the reputation of his late father, and his steadfast determination to fight the Americans. He has not been seen in public for a week-and-a-half, and his whereabouts are unknown.

Grand Ayatollah Ali al-Sistani is the most senior Shia cleric in Iraq, and is considered a leader for Shias worldwide. He defused the last Sadrist uprising, which began in April, by brokering a ceasefire that lasted for several months. But the latest rebellion started in early August, the day before Mr. al-Sistani had to leave the country for medical treatment in London.

He has had minor surgery for a heart condition. Aides say he is recovering well.