BAGHDAD, Iraq — U.S. and Iraqi forces arrested one of Muqtada al-Sadr's top aides today 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 today 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.

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.

"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.