U.S. and Iraqi forces arrested one of Muqtada al-Sadr's top aides Friday in Baghdad, his office said, as pressure increased on the radical Shiite cleric's militia ahead of a planned security crackdown in the capital.

Al-Sadr said in an interview with an Italian newspaper published Friday that the crackdown had already begun and that 400 of his men had been arrested. La Repubblica also quoted him as saying he fears for his life and stays constantly on the move.

The raid came as Defense Secretary Robert Gates began his second trip to Iraq in less than a month, arriving in the southern city of Basra to consult with British and other allied commanders.

Sheik Abdul-Hadi al-Darraji, al-Sadr's media director in Baghdad, was captured and his personal guard was killed, according to another senior al-Sadr aide.

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"We strongly condemn this cowardly act," said Sheik Abdul-Zahra al-Suweiadi.

The U.S. military said special Iraqi army forces operating with coalition advisers captured a high-level, illegal armed group leader in Baladiyat, an eastern neighborhood near al-Sadr's stronghold. It did not identify the detainee, but said two other suspects were detained by Iraqi forces for further questioning.

Prime Minister Nouri al-Maliki has pledged to crack down on Shiite militias as well as Sunni insurgents in a planned security operation. His reluctance to confront the Mahdi Army of al-Sadr, his political backer, has led to the failure of previous efforts to stem sectarian violence in Baghdad.

In the interview with La Repubblica, al-Sadr said his militias would not fight back during the Muslim holy month of Muharram, saying it was against the faith to kill at that time.

"Let them kill us. For a true believer there is no better moment than this to die: Heaven is insured," he was quoted as saying. "After Muharram, we'll see."

The Muharram starts Friday for Sunnis and Saturday for Shiites.

Al-Sadr said he is being targeted.

"For this reason, I have moved my family to a secure location. I even have had a will drawn up, and I move continuously in a way that only few can know where I am," he was quoted as saying by Repubblica.

Militia commanders have said the Shiite prime minister has stopped protecting the fighters under pressure from Washington and have described pinpoint raids in which at least five top commanders of similar standing were captured or killed in recent months.

The U.S. military accused the main suspect captured Friday of having ties with the commanders of so-called death squads, 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.

The suspect was detained "based on credible intelligence that he is the leader of illegal armed group punishment committee activity, involving the organized kidnapping, torture and murder of Iraqi civilians," according to the military statement.

It also said he was reportedly involved in the assassination of numerous Iraqi security forces and government officials.

"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," the statement read, adding he was believed to be affiliated with Baghdad death squad commanders, including Abu Diraa, a Shiite militia leader who has gained a reputation for his brutality.

Al-Suweiadi did not give more details, but another official in al-Sadr's office said al-Darraji was captured during a 2 a.m. raid on a mosque in Baladiyat, less than a mile from a U.S. base.

The official and an Iraqi police officer, who spoke on condition of anonymity because of security concerns, also said one of the mosque's guards was killed in a firefight during the raid that damaged the mosque walls, while four other people who were with the sheik were arrested.

Abdul-Razzaq al-Nidawi, an al-Sadr aide in the Shiite holy city of Najaf, demanded that al-Darraji and other detainees from the cleric's movement, 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."

Gates, who met earlier with the leaders of Saudi Arabia and Qatar, said they had expressed concern about whether al-Maliki can deliver on his promises to rein in the violence and "quite frankly, these are reservations that have been expressed in Washington, and we will be watching."

Highlighting the challenges, a rocket struck a British military base late Thursday in Basra, wounding six soldiers, spokeswoman Capt. Katie Brown said, hours before Gates arrived.

Britain, which has the largest troop contingent among the U.S. allies, with about 7,000 soldiers in the Basra area, is planning to withdraw a large portion of them this year.

A roadside bomb killed one U.S. soldier and wounded three others in an attack against a patrol that was escorting a convoy in northwestern Baghdad, the military said Friday.

U.S. and Iraqi forces are gearing up for a major neighborhood-by-neighborhood sweep to quell the spiraling violence in the capital.

At least 3,030 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count.