Anti-American cleric Muqtada al-Sadr called Sunday for the Iraqi government to open a "fair" investigation into last week's deadly clashes surrounding a Shiite religious celebration in Karbala, which many have blamed on his militia.

Al-Sadr's followers threatened to take unspecified measures if the government refuses the demand. They also warned they would halt a six-month suspension of Mahdi Army attacks if U.S. and Iraqi forces continue operations against militia members.

Al-Sadr announced the surprise six-month suspension of Mahdi Army activities last Wednesday following the Karbala bloodshed, which saw more than 50 people killed and hundreds injured, in an apparent bid to deflect criticism.

Security officials said the trouble started when members of al-Sadr's Mahdi Army confronted mosque guards affiliated with the Badr Organization, allied with the Supreme Islamic Council of Iraq.

Al-Sadr, who has denied that the Mahdi Army provoked the confrontation, urged an investigation and added a threat to take matters into his own hands if the government does not act.

"We warn the government and the concerned sides that if a fair investigation is not opened, we will be forced to take different measures," he said in a statement read to The Associated Press by spokesman Sheik Salah al-Obeidi. He refused to elaborate on what the measures may be.

Though the Iraqi government and U.S. commanders have praised al-Sadr's move to stand down his militia, security forces have been keeping the Mahdi Army under pressure, saying they are focusing on breakaway factions believed to be receiving weapons, training and money from Iran — a charge that Iranians deny.

Before dawn on Sunday, police in Nasiriyah, about 200 miles southeast of Baghdad, said they arrested six suspects in coordinated raids on several houses believed to be used by Mahdi Army militiamen responsible for recent violence in the area.

The U.S. military conducted several other raids in Baghdad on Saturday that targeted Mahdi Army factions, arresting 11 suspects.

In Kufah, al-Obeidi told Associated Press Television News that if attacks on al-Sadr followers continue, the cleric would end his order to freeze militia activities for six months.

In a late interview Saturday night, U.S. Ambassador Ryan Crocker said that during recent talks with the Iranians in Baghdad, the U.S. and Iraqi sides had demanded an end to Tehran's alleged support for the militias.

"We want an end to Iranian support to some fanatical militias that are fighting coalition and Iraqi forces. We also want an end to the shelling of the Green Zone, we want an end to training those militias. This is what we and the Iraqi government want and until now there has been no positive response on the ground," he said in the interview on state television, speaking in Arabic.

He added that it was "important for everyone to understand" that the U.S. was trying to solve its differences with Iran and its ally Syria through political and diplomatic means.

"I returned to Iraq at the beginning of this year because I was optimistic about the future of Iraq. Six months later, I am still optimistic because you have made tangible progress toward a democratic Iraq," he added.

Crocker, who is due to report mid-month to U.S. Congress on the situation in Iraq with Gen. David Petraeus, said he did not anticipate any major changes to American policy.

"Since 2003, there has been a stable policy by the American administration and I don't think there will be a fundamental or quick change in the American policy or stand on Iraq," he said.

He said the ongoing security operation in Baghdad has brought tangible improvements in public safety but added there was a "need for common resolutions by everybody — Sunnis, Shiites and Kurds."