BAGHDAD (Reuters) - - A car bomb killed 26 people in a Shi'ite militia stronghold in east Baghdad on Saturday, hours after Sunni Muslims began the fasting month of Ramadan, which U.S. commanders warn may see a rise in sectarian bloodshed.

State television, quoting the Defence Ministry command, flashed the arrest of a man it said was the leader of one of the most militant Sunni groups, Ansar al-Sunna, which has been allied with al Qaeda. However, spokesmen for the ministry and the U.S. military could not confirm the arrest.

Police said the car bomb blast near a fuel station in Sadr City, one of the deadliest in recent weeks, also wounded at least 29 people. It was not immediately clear whether the vehicle was parked or driven by a suicide attacker, they said.

U.S. and Iraqi troops have mounted a major security crackdown since last month in an effort to clamp down on violence that they warn risks destroying the four-month-old national unity government through all-out sectarian civil war.

Sadr City, slum stronghold of the movement of radical Shi'ite cleric Moqtada al-Sadr, had been spared much of the violence until recent months when Sunni accusations have grown that his Mehdi Army militia is among those behind thousands of "death squad" killings.

Many Sunnis, a majority in the Arab world but a disaffected minority in Iraq, began to observe the Ramadan fast on Saturday, a timing dependent on sightings of the moon. Clerics from Iraq's Shi'ite Muslim majority have yet to announce the start of the month but are likely to do so in the next day or two.

A source in the office of Grand Ayatollah Ali al-Sistani, the leading Shi'ite cleric in Iraq, said it expected to announce late on Sunday that Ramadan would start on Monday.

U.S. commanders have said that, as in the past three years, they anticipate an increase in violence during Ramadan, when Muslims fast during daylight and spend more time in prayer.

"MILITANT CAPTURED"

Iraqiya state television said Muntasir al-Jibouri, which it described as the leader of Ansar al-Sunna, and two aides, had been captured in Muqdadiya, a town in Diyala province northeast of Baghdad that has seen heavy al Qaeda activity.

U.S. forces killed al Qaeda in Iraq leader Abu Musab al-Zarqawi in that region in June and Iraqi and U.S. officials have announced the arrests of other militants there since then.

The written newsflash quoted the armed forces high command, but an official there said he too could not confirm the report.

Ansar al-Sunna emerged out of an Islamist movement called Ansar al-Islam, which was based in autonomous Kurdish northern Iraq during the Sunni-dominated secular rule of Saddam Hussein.

Ansar al-Sunna came to prominence by releasing film of beheadings of foreign hostages and also claimed a suicide bombing at a U.S. military canteen in Mosul in December 2004 that killed more than 20 people, including 19 Americans.

Apparent successes against Sunni militants who have attacked occupying U.S. forces and the Shi'ite-led government for the past three years, have come at a time when U.S. commanders warn of a growing risk of sectarian civil war involving Shi'ite and Sunni armed groups. Dozens of people die in Baghdad every day.

The U.S. commander of troops in the capital said on Friday that U.S.-trained Iraqi soldiers do not want to leave other parts of their country to serve in Baghdad, leaving security efforts in the capital short by 3,000 Iraqi troops.

"I would tell you I need more Iraqi security forces," Major General James Thurman said.

Six Iraqi army battalions -- roughly 500 soldiers each -- that he has requested to reinforce Baghdad have not been provided by the Iraqi government, he added, because the troops do not want to go to the violent city.

"The government is trying to come to grips with the security needs," Thurman said.

"Baghdad's security hinges on the capabilities of the Iraqi security forces," Thurman added, arguing against putting more U.S. troops in Baghdad.