BAGHDAD — Iraqi security forces clashed with Shiite militia fighters southeast of Baghdad today, the second day of fighting that killed at least two police officers and two gunmen, police said.

The fighting in the city of Kut broke out after factions of the Mahdi Army militia attacked checkpoints around the city amid a crackdown by Iraqi troops.

A joint American-Iraqi operation also targeted a Shiite militia stronghold in the volatile city of Diwaniyah, 80 miles south of Baghdad, and at least 12 suspected fighters were detained, local police said. The Americanmilitary had no immediate comment.

The fighting underscored rising tensions between rival militia factions battling for control of the oil-rich southern Shiite heartland with an eye toward the eventual withdrawal of American-led forces.

The Mahdi Army militia is nominally loyal to the radical cleric, Muqtada al-Sadr, who renewed a six-month truce in February, but its fighters have been involved in clashes. America has not accused Mr. al-Sadr in the violence, blaming instead rogue militiamen who ignore his cease-fire order.

But American and Iraqi officials have been cracking down on Mr. al-Sadr’s followers, angering many in the movement who complain the security forces have been infiltrated by rival factions.

Iraqi reinforcements were sent to Kut four days ago to wrest control of a militia stronghold controlled by Mahdi Army fighters who had become increasingly brazen in recent weeks in their attacks on security forces, police said.

The fighting in Kut, about 100 miles southeast of Baghdad, began when militiamen attacked checkpoints around the city yesterday. At least two police officers and two gunmen had been killed since then, said Major General Abdul-Karim Khalaf of Iraq’s Interior Ministry.

In Baghdad, American troops fought with Shiite gunmen today in a southwestern neighborhood, police said, speaking on condition of anonymity because they were not authorized to release the information. The military did not immediately comment and no other details were available.

The American military released 13 detainees who were welcomed home at a ceremony in the former Sunni insurgent stronghold of Azamiyah in northern Baghdad. It was the latest in a series of releases meant as a goodwill gesture to promote reconciliation with minority Sunnis who have turned against Al Qaeda in Iraq.

One of those freed, Muntasir Abdul-Mahdi, said he had been held at Camp Bucca for a year after being arrested by American troops.

“They said that they would make inquiries, but I was there for one year,” he said. “I am innocent. They released us because we are innocent.”

American officials have touted the sharp decline in violence over the past year as a sign the Bush administration is beginning to show success in a conflict widely unpopular with the American public.

According to the American military, attacks have fallen about 60% since early last year, when President Bush ordered in about 30,000 American reinforcements to curb a wave of sectarian killings that had Iraq teetering on civil war.

But American commanders caution that Iraq remains far from secure and say the security gains are fragile because of political disputes among the country’s rival Shiite, Sunni, and Kurdish communities.