BAGHDAD — Iraqi Prime Minister Nouri al-Maliki issued his strongest warning yet to radical Shiite cleric Muqtada al-Sadr to disband his Mahdi Army militia or face political isolation.

The Sadrists fired back today, saying a move to ban them from elections would be unconstitutional.

The U.S. military, meanwhile, said two more soldiers died in roadside bombings on Sunday, raising the day's American death toll to at least five. The announcement comes as the top two U.S. officials in Iraq prepare to brief Congress on the prospects for the eventual withdrawal of American troops.

Al-Maliki, himself a Shiite, told CNN Sunday that al-Sadr's followers would not be allowed "to participate in the political process or take part in upcoming elections unless they end the Mahdi Army."

He was referring to provincial elections expected in the fall that are likely to redistribute power in Iraq. The Sadrists have accused al-Maliki's government and rival parties of trying to diminish their standing ahead of the vote.

The prime minister, who took office in May 2006 with al-Sadr's support but later broke with the powerful cleric, had in the past repeatedly promised to disband militias but his comments on CNN signaled the first time he publicly singled out the Mahdi Army.

Senior Sadrist lawmaker Baha al-Aaraji called for calm but said the prime minister had no constitutional right to interfere with the elections.

"The Supreme Electoral Commission is the one to decide, not the prime minister, so the prime minister should not interfere in the work of this commission," al-Aaraji said Monday at a news conference.

Lawmakers and officials involved in the effort to isolate the Sadrists politically have told The Associated Press that the first step would be adding language to a draft election bill banning parties that operate militias from fielding candidates in the provincial balloting due this fall.

The government intends to send the draft to parliament within days and hopes to win approval within weeks, they said Sunday.

Such a move risks a violent backlash by al-Sadr's Shiite militia. But if it succeeds it could cause a major realignment of Iraq's political landscape.

A roadside bomb killed a U.S. soldier Sunday in eastern Baghdad, the military said in a statement issued Monday. It did not specify the location but the area includes Sadr City and other predominantly Shiite neighborhoods.

Another American soldier was killed Sunday by a roadside bomb north of the capital in Diyala province, the military said separately. That raised to two the number of U.S. troops killed in that blast.

Suspected Shiite militants lobbed rockets and mortar shells into the U.S.-protected Green Zone and a military base elsewhere in Baghdad on Sunday, killing three American troops and wounding 31, officials said.

At least 4,020 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count.

The inability of the Iraqi forces to curb the militias has cast doubt on their ability to take over their own security. The top American officials in Iraq — Gen. David Petraeus and Ambassador Ryan Crocker — are to start briefing Congress Tuesday on the prospects for further reductions in the U.S. troop presence in Iraq.

Al-Sadr has called for a "million-strong" anti-U.S. demonstration on Wednesday in Baghdad to protest the fifth anniversary of the capture of the Iraqi capital by invading U.S. troops.

In his remarks, al-Maliki said he had a wide spectrum of political support for his efforts against the Mahdi Army.

"Solving the problem comes in no other way than dissolving the Mehdi Army," Maliki said. "We have opened the door for confrontation, a real confrontation with these gangs, and we will not stop until we are in full control of these areas."

But he acknowledged that Iraqi security forces faced obstacles but he said he was surprised the militia fighters had not mounted a fiercer response.

"Confronting the militias does still need more effort," he said. "Our readiness is not at full level yet, but what is happening in Sadr City is still less than what people expected the militias to do."

Hospital officials in Baghdad's Sadr City said three more civilians were killed and 36 others wounded in overnight clashes, raising to at least 19 the number of civilians who died in the clashes that broke out Sunday.

It was the fiercest fighting since al-Sadr ordered a cease-fire a week ago.

On Monday, a large explosion demolished a building in the southern city of Basra, killing at least three militants and wounding four others, Iraqi security forces said.

An Iraqi police official and witnesses said the building was destroyed in an air strike. The official spoke on condition of anonymity because he was not authorized to release the information.

British military spokesman Maj. Tom Holloway said no British, U.S. or Iraqi forces were involved.