Iraq has arrested suspects in the assassination of Muslim cleric al-Sader, left RELATED VIDEO Reporter Rula Amin went on a tour of Nasiriya organized by Iraqi authorities Windows Media 28K 80K

In this story: Iraq, U.S. trade charges of culpability Sketch of other suspect released Regime seeks to dispel riot reports U.S. sees 'signs of domestic instability' RELATED STORIES, SITES

BAGHDAD, Iraq (CNN) -- Iraq has arrested some suspects in the assassination of a top Shiite Muslim cleric and is seeking more, the country's foreign minister said Tuesday as Baghdad moved to buttress its assertions that it was not involved in the killing.

Mohammed Saeed al-Sahhaf also suggested for the first time that the United States may have played a role in the killing of Grand Ayatollah Mohammed Sadiq al-Sader last week.

"Fortunately, some of those who took part in the assassination were caught, and the authorities are interrogating them," al-Sahhaf told a news conference Tuesday. "You will all know soon the true facts surrounding the death of the great scholar, martyr al-Sader."

"I surely do not rule out that they (the Americans) have had a finger in this crime to create a sedition in Iraq that serves their criminal and colonial scheme to interfere in Iraq's internal affairs," al-Sahhaf said.

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Al-Sahhaf did not cite specific evidence but spoke of America's declared support for Iraqi opposition groups seeking to overthrow the regime in Baghdad.

In Washington, Deputy State Department spokesman James Foley dismissed the Iraqi assertions of U.S. involvement in the assassination as "lies."

"Sources within the opposition as well as religious leaders outside Iraq have attributed the assassination to the regime," Foley said. "This is by all accounts another act of brutal repression on the part of the Iraqi regime."

"The ayatollah had reportedly been prevented from leading Friday prayers, he had been interrogated and threatened by security forces of the regime, all of which point to the possibility of regime involvement," Foley added.

Later, state-run Iraqi television broadcast an artist's sketches of a man police are seeking in connection with the killing of al-Sader and two of his sons. The man, who appeared to be an Arab in his mid-20s, had a mustache and closely trimmed beard and was depicted both with a traditional headdress and bareheaded.

It did not give the man's name, profession, nationality or any other personal details but told Iraqis to call a phone number to inform police if they were able to identify the man.

Gunmen shot and killed the cleric and his sons as they were driving home Friday in Najaf, a city holy to Shiites in southern Iraq.

Al-Sader was the third Shiite religious leader to be assassinated in Iraq in the past year, and Iran's spiritual leader, leading Lebanese Shiite clerics and Iraqi dissidents have all accused the Iraqi government of planning the most recent killings.

Trying to counter reports of riots following al-Sader's death, the Iraqi government took reporters Tuesday on a tour of Nasiriya, 225 miles south of the capital of Baghdad.

Reporters were taken inside the governor's offices and past the security headquarters, but they saw no sign of fires that opposition groups had said occurred at those sites in the Shiite city.

On the streets, life appeared normal. Stores were opened, and people were going about their usual business. The only break from routine, was a wailing siren, warning people of Western warplanes, flying over their town.

But CNN has confirmed that some clashes did take place in one of Baghdad's suburbs.

Foley, in Washington, said the number of casualties in "widespread" demonstrations throughout Iraq protesting al- Sader's killing is difficult to ascertain, but Iraqi opposition sources -- whom he said have been reliable in the past -- had reported 25 killed over the weekend.

That assertion was echoed at the Pentagon, where spokesman Ken Bacon said: "There are signs of domestic instability in Iraq. We don't know how seriously to take them, but in the last couple of weeks, the last couple of days following the assassination of the Grand Ayatollah al-Sader, there have been demonstrations and unrest in Iraq."

The Tehran-based Supreme Council for the Islamic Revolution in Iraq said in a statement dated Sunday that 18 people were killed in Nasiriya and the town was no longer in government hands. It said 300 were killed in unrest in a Baghdad Shiite district. Other groups reported unrest in Kerbala.

In Iran on Tuesday, Iraqis protesting the murder of al-Sader broke into the Iraqi Embassy in rowdy demonstrations. The death also has prompted demonstrations in Jordan, Lebanon and Syria.

Shiite Muslims comprise 65 percent of Iraq's 22 million people. There have been recurrent reports of unrest between them and the Sunni-led regime, mostly in the south where Iraqi troops crushed a Shiite-led rebellion after the Gulf War.

Military Affairs Correspondent Jamie McIntyre, Producer Sharona Schwartz, Reporter Rula Amin and Reuters contributed to this report.