A gun battle erupted this evening in the holy Iraqi city of Najaf after US troops approached the house of militant Shi'ite cleric Moqtada al-Sadr.
They said members of Sadr's Mehdi Army militia exchanged fire with the soldiers. Sheikh Mahmoud al-Sudani, a spokesman for Sadr in Baghdad, said the soldiers had surrounded Sadr's house.
Sadr launched an uprising against US-led forces in April, and hundreds were killed in weeks of fighting before truces were agreed in Shi'ite areas of Iraq .
The US military says an Iraqi arrest warrant has been issued for Sadr in relation to the killing of a rival cleric in Najaf last year.
But during truce negotiations earlier this year, Iraqi officials said Sadr would not face arrest.
Meanwhile a series of attacks on Christian churches in Iraq has been blamed on another prominent militant,
The co-ordinated car bombings at churches killed at least 11 people yesterday, according to Iraqi authorities.
Muslim leaders including top Shi'ite cleric Ayatollah Ali al-Sistani condemned the car bombings, which were timed for evening services in Baghdad and the northern city of Mosul.
The attacks were the first on the minority Christian community's churches since the start of a 15-month insurgency.
"There is no shadow of a doubt that this bears the blueprint of Zarqawi," said national security adviser Mowaffaq al-Rubaie. "Zarqawi and his extremists are basically trying to drive a wedge between Muslims and Christians in Iraq . It's clear they want to drive Christians out of the country," he said.
The Jordanian-born militant has claimed responsibility for many major car bombings in Iraq since Saddam Hussein was ousted last year and also the killing of several foreign hostages among dozens seized in recent months.
A group linked to Zarqawi executed a Turkish hostage today shown in gruesome detial on an Islamist Website.
In response to the killing and a wave of kidnappings of Turkish drivers, a Turkish truckers' group said it would stop transporting goods to US forces in Iraq .
al Qaeda ally Abu Musab al-Zarqawi.