Baghdad — A senior aide to the radical anti-American cleric Muqtada al-Sadr was assassinated in Najaf as he returned home from Friday Prayer, raising the likelihood that tensions would climb still higher between al-Sadr's loyalists and the Iraqi government forces they have been battling.

The police declared a curfew in Najaf, the holiest Shiite city in Iraq, and deployed reinforcements on the streets, fearing a backlash after the murder of the aide, Riyadh al-Nouri.

Security officials there said al-Nouri, who was related to al-Sadr's family by marriage, was ambushed by unknown gunmen outside his house in the Najaf neighborhood of al-Adala after leaving prayers in the nearby town of Kufa.

Iraqi security forces carried out a major offensive in Basra last month, but it was widely criticized as poorly planned and the government failed in its attempt to disarm Shiite militias, particularly the Mahdi Army founded by al-Sadr. In recent days, the two sides have been fighting heavily in al-Sadr's eastern Baghdad stronghold, Sadr City.

Within hours of al-Nouri's killing, al-Sadr's office in Najaf issued a statement laying blame at “the hands of the occupiers and their tails,” meaning the United States and Prime Minister Nouri Kamal al-Maliki's government.

The statement added: “I promise before God and the Iraqi people that I will never forget this blot, and the occupier will never feel safe in our land, while I am alive.” Al-Nouri's sister is married to al-Sadr's brother, Sadrist officials said.

Still, a spokesman for al-Sadr, Salah al-Obeidi, urged the cleric's followers to be calm and “not to be dragged into others' plots,” The Associated Press reported.

Shortly after the killing, a rocket, apparently fired at the Green Zone, struck the Palestine Hotel in central Baghdad, the site of a coordinated suicide truck bombing in 2005 that killed at least six Iraqis and wounded more than 40.

The missile on Friday punched through a foot of concrete and destroyed a first-floor room on the hotel's east side. Hotel officials, guards and neighbors said no one was wounded in the attack, but local police officials later claimed that three people had been killed.

Al-Maliki's office issued a statement condemning al-Nouri's killing and ordered an investigation. Al-Maliki called on security forces to “be ready to resist anyone who tries to disturb the security situation in the holy city of Najaf.”

Al-Nouri's brother Ahmad Jassem al-Nouri said Friday that the assassination was carried out by gunmen using two cars and a motorcycle.

“It is a planned operation against the Sadr movement, and it is a message to Mr. Muqtada,” he said. “When they kill someone very close to him, that means they are after him.”

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