U.S. military commanders in Iraq say they believe the surge of violence this week in the southern, mostly-Shi'ite city of Basra is related to the growing hostility between two Shi'ite entities, whose militias are fighting for dominance in parts of Iraq. Military officials fear the emerging intra-Shi'ite power struggle is posing as big a threat to Iraq's security as the country's Sunni-led insurgency.

For months, U.S. military officials have been keeping track of a growing number of clashes between rival Shi'ite militia groups, one opposed to Iranian influence in Iraq, and one with open ties to Iran.

The Mahdi Army is a militia group loyal to firebrand cleric Moqtada al-Sadr, which staged two bloody rebellions against U.S. forces last year. Moqtada al-Sadr touts himself as a nationalist, vehemently opposed to both the U.S. military presence in Iraq and Iranian meddling in Iraqi affairs.

Mr. Sadr gets most of his support from the 2.5 million poor and disgruntled Shi'ite residents of Sadr City, an impoverished Baghdad neighborhood. It is named after the cleric's father, a revered grand ayatollah, who was assassinated in 1999 on the orders of Saddam Hussein.

The Badr Organization is the armed wing of the Supreme Council for Islamic Revolution in Iraq, or SCIRI, the main Shi'ite party in Iraq's coalition government. For decades, the party and the Badr group operated out of neighboring Iran in their fight against Saddam Hussein's regime. Party leaders and Badr fighters returned to Iraq after the U.S. invasion in 2003, but are believed still to be receiving Iranian support.

After months of low-level skirmishes, the Mahdi Army and the Badr Organization openly clashed last month in the Shi'ite holy city of Najaf. Six people were killed and dozens wounded. The incident sparked a second day of violent clashes across the Shi'ite heartland of central and southern Iraq.

U.S. Army Colonel Joe DiSalvo, the commander of the Second Brigade, Third Infantry Division, which oversees Sadr City, says it is difficult to know how many hardcore fighters are in Mr. Sadr's Mahdi Army. Estimates range from two to several thousand. The Badr Organization is believed to number about 20,000.

While most of the Badr fighters are well armed and received professional training in Iran, the gritty streets of Sadr City have been the main training ground for the Mahdi Army militia.

Colonel DiSalvo describes the Mahdi Army as loosely organized, and made up mostly of poor, uneducated Iraqis, with little military training or discipline.

"They are very multi-cellular organized, so once a directive is given, it gets very difficult to adjust tactics, just because it will come from one guy, go out to 27 different cells," he said. "Their communication is not that good, and if they want to go left instead of right after an hour, it is going to take a long time for them to catch up."

Twenty-six-year-old Sadr City auto mechanic Hassan Shamal, says, the Mahdi Army is a trusted force, and now runs all security checkpoints inside Sadr City, not the Iraqi police.

Mr. Shamal says people in Sadr City feel safer with the Mahdi Army in charge. He says no one there trusts the Baghdad police, because people believe they have been infiltrated by members of the Badr Organization.

There have been numerous complaints from both Sunni Arabs and Shi'ites about what critics say is the Badr Organization's increasing influence throughout the Ministry of Interior, which controls Iraq's police force.

A group of Shi'ite men, who applied to the ministry for jobs as policemen, recently spoke to VOA, but refused to be identified by name, believe they were turned away because the current interior minister, Bayan Jabr, had already filled the positions with Badr fighters. Mr. Jabr is a former high-ranking Badr officer and a member of the SCIRI party.

Recently, there have been signs that the Mahdi Army may be gaining the upper hand over the Badr Organization in key Shi'ite areas, such as the southern city of Basra, where police sources say the Mahdi Army is effectively in charge of security.

Police sources there say a raid by the British Army earlier this week to free two of its men held captive took place in a building that houses Basra's secret internal affairs department. Until earlier this year, the department was under Badr control, but now, senior police sources say, it is in Mahdi Army hands.

The existence of militias with conflicting loyalties is complicating Iraq's efforts to establish an all-inclusive, functioning central government.

The two main Kurdish parties have the strongest militia, with more than 100,000 fighters, and argue they are needed to protect Kurdish interests.

Three months ago, the head of the Badr-affiliated SCIRI party, Abdel Aziz al-Hakim, stressed the importance of maintaining the militia.

Colonel Joe DiSalvo says, given such attitudes, militias are not likely to be dismantled any time soon.

"The government has got to come to grips with how are they going to legitimize militias, or not. And they have to practice what they preach, too," he explained. "If they say militias are not authorized, and yet their own personal protection is from their political party's militia, and not from the Iraqi security forces, then we are never going to solve this militia problem."

The U.S. military says it is bracing for another possible showdown between Badr and Sadr militias ahead of the October 15 referendum on the country's draft constitution.

The dominant Shi'ite SCIRI party and its Badr militia strongly support a clause in the constitution, which endorses establishing a semi-autonomous zone in Iraq's Shi'ite-dominated, oil rich south.

Moqtada al-Sadr, whose power base is in Baghdad, not the south, has sided with Sunni Arabs in denouncing federalism, and has hinted that he might support a Sunni-led effort to defeat the constitution at the polls.

Such a move by the radical cleric could shatter Shi'ite unity in Iraq. On Thursday, the country's most revered and powerful cleric, Grand Ayatollah Ali al-Sistani, endorsed the draft constitution, and instructed his senior aides to promote a "yes" vote among Shi'ite Muslims.