The top U.S. commander in Iraq, General David Petraeus, has condemned the assassination of a senior aide to radical Shi'ite cleric Moqtada al-Sadr in the Iraqi holy city of Najaf. VOA correspondent Meredith Buel has details from the Pentagon.

General Petraeus told a small group of reporters at the Pentagon that the killing of Sadr advisor Riyadh al-Nouri potentially threatens weeks of calm in Najaf.

"The assassination in Najaf is obviously a cause for significant concern. I am sure that there are numerous calls for calm, for restraint in response. I know that there has been a curfew established already in Najaf. I am sure there will be pledges to bring to justice whoever it was that carried out this murderous action," he said.

Police say Nouri was killed when gunmen fired on his car near his home while returning from prayer services.

He was one of Sadr's closest aides. Nouri's sister is married to one of Sadr's brothers.

A spokesman for Sadr in Najaf said the cleric is calling on his supporters for calm, but is blaming what he called occupying forces and those working with them for the assassination.

General Petraeus denied U.S. forces were involved. "I have no idea what the basis of that would be. We do not have forces operating in Najaf. This is an act that we condemn as do all other Iraqi leaders and coalition leaders," he said.

Tension between Sadr's militia and Iraqi government forces recently exploded into violence when Prime Minister Nouri al-Maliki ordered a crackdown on Shi'ite militias and armed gangs in the southern city of Basra last month.

Sadr's militia responded with attacks in Basra and Baghdad, showering the U.S.-controlled Green Zone with rockets and mortars.

American and Iraqi officials insist the Basra crackdown was not aimed at Sadr's political movement, but at criminals and Iranian-backed splinter groups.

General Petraeus called on all parties in Najaf to continue efforts to keep the holy city peaceful. "There has actually been very constructive dialog and engagement between the different parties in Najaf. Between the provincial governor and council members, the police chief, various security force leaders and of course it is in everyone's interest to maintain the peace in the holy city of Najaf," he said.

Clashes have continued in Basra and Baghdad, despite Sadr's order for his militiamen to stand down.

Sadr is believed to be in Iran and has not been seen in public for nearly a year.