· Backlash from fellow Shias after three days of clashes · Al-Sadr orders days of mourning and shuts office

The Iraqi militia leader Moqtada al-Sadr, called a six-month truce yesterday after fighting with a rival Shia Muslim group in the holy city of Kerbala left more than 50 dead.

A spokesman for the Mahdi army claimed they would lay down their weapons for six months and, during this time, would attack neither other Shia groups nor the US army.

Mr Sadr, who has thousands of armed men at his command, has called truces before but these have been short-lived.

Although US forces will welcome any respite, most of the attacks they face in the centre and north of Iraq are from Sunni nationalist groups and al-Qaida in Iraq.

A US military spokesman, Lieutenant-Colonel Christopher Garver, said any organisation that worked towards a peaceful Iraq would be appreciated.

Since President George Bush announced in January he was to send an extra 30,000 US troops to Iraq, the Mahdi army has reduced the number of attacks on US forces. Mr Sadr was reported by the US to have sought temporary sanctuary in Iran for a time, though he denied it.

Even when US forces went into Sadr city, a slum area of Baghdad that is the stronghold of Mr Sadr, the predicted bloodbath did not materialise and the Mahdi army has maintained a low profile.

Mr Sadr's truce appears to be in response to a backlash by fellow Shias over three days of Shia in-fighting at a time when hundreds of thousands of pilgrims are in Kerbala. The fighting forced many pilgrims to flee.

At least 52 died and 279 were wounded on Tuesday in the fighting between the Mahdi army and the Supreme Iraqi Islamic Council. A curfew has been imposed on the city.

Hazim al-Araji, an aide to Mr Sadr, read out a statement to Reuters saying the militia would suspend its operations "to restructure it in a way that will preserve its principles". He said Mr Sadr had ordered all his movement's offices shut for three days of mourning.

The restructuring is a hint he is trying to distance himself from elements in the Mahdi army being blamed for taking violence close to two holy sites, the Imam Hussein and Imam Abbas shrines. He can claim that parts of the Mahdi army are outside his control. Another aide suggested as much when he said the aim was to remove bad members "working for their personal interests ... to hurt the Mahdi army's reputation". Both the Mahdi army and the SIIC have links with their co-religionists in Iran, though the extent of these relationships are a matter of dispute.

Mr Bush has accused the Iranian government this year of meddling in Iraq, including providing weapons to insurgents, and ordered US diplomats and military to adopt a more forceful stance towards Iranians in Iraq.

Iran protested yesterday after eight Iranians, including two with diplomatic credentials, were arrested by US forces at a checkpoint in Baghdad on Tuesday as they were heading towards the Sheraton Ishtar hotel. They were released after the Iraqi government interceded on their behalf.

The Iranians had been accompanied by Iraqis who were providing protection. US troops seized three weapons from the cars for which there had been no permits.

The Iranians were apparently part of an official delegation to discuss cooperation on supplying electricity to villages along the Iraqi border.

Hoshyar Zebari, the Iraqi foreign minister, told the BBC: "After we intervened with the embassy and explained the situation that they were here on legal grounds on a legal basis and they have their visas, their credentials were established, then they were released."

He added that the detention had nothing to do with Mr Bush's speech that day in which he ramped up the threat to Iran.

In Tehran, the Iranian foreign ministry summoned the Swiss diplomat representing American interests in Iran to protest. The US has no diplomatic links with Iran.