US and Iraqi forces have arrested one of the top aides of the radical Shia cleric, Hojatoleslam Moqtada al-Sadr, as the Iraqi Government prepares to launch a crackdown on militias accused of enflaming the sectarian violence which has plagued the country.

The pre-dawn arrest, which came as US Defence Secretary Robert Gates embarked on his second trip to Iraq in less than a month, highlighted the tensions between the two nations, with an adviser to the Iraqi Prime Minister insisting that the Government had received no prior notice of the raid.

"Coordination with Iraqi political leadership is needed before conducting such an operation that draw popular reactions," said Sadiq al-Rikabi, adding that the arrest was not part of the government’s latest initiative to put an end to the militia-driven bloodshed.

Sheikh Abdul-Hadi al-Darraji, Hojatoleslam al-Sadr's media director in Baghdad, was captured in the early hours in a mosque in the eastern neighbourhood of Baladiyat, according to the cleric’s aides, who said that a guard was killed in the "cowardly" raid.

The US military did not name the detained man but said special Iraqi army forces operating with coalition advisers had captured a high-level, illegal armed group leader in Baladiyat. It added that two other suspects were detained by Iraqi forces for further questioning.

The comments from Prime Minister Nouri al-Maliki’s adviser appeared to show the divergence between the US and Iraqi authorities on how best to deal with the Shia militias which have been blamed for many of the killings that have left dozens of bodies, often showing signs of torture, on the streets of Baghdad.

Mr al-Maliki, whose Shia-led government counts Hojatoleslam al-Sadr and his followers as crucial allies, has been criticised for his apparent reluctance to confront the influential cleric and to disarm his powerful al-Mahdi Army.

But with his latest pledge of a crackdown widely viewed as a last-ditch hope, the Prime Minister has been under increasing pressure to act. Earlier this month, he said that 400 members of the militia had been arrested – a claim confirmed by Hojatoleslam al-Sadr in a newspaper interview.

US forces accused the arrested suspect of having ties with the leaders of Shia death squads. One such figure is said to be Abu Diraa, a militia commander who has gained a reputation outstanding brutality and is accused of killing Iraqi and US soldiers as well as several Sunnis.

"The suspect allegedly leads various illegal armed group operations and is affiliated with illegal armed group cells targeting Iraqi civilians for sectarian attacks and violence," said a US military statement.

Abdul-Razzaq al-Nidawi, an aide to Hojatoleslam al-Sadr in the Shia holy city of Najaf, demanded that Mr al-Darraji and other detainees be released and called for demonstrations after the weekly Friday prayer services.

"America is playing with fire and our patience is beginning to fade," he said. "This savage barbarian act will not pass peacefully."

Mr al-Maliki today came under further criticism from Washington, as Mr Gates, who met earlier with the leaders of Saudi Arabia and Qatar, said they had expressed concern about whether the Prime Minister could deliver on his promises to rein in the violence.

"Quite frankly, these are reservations that have been expressed in Washington, and we will be watching," he warned.

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