BAGHDAD - A bomb killed 34 people in Baghdad's Sadr City Shi'ite slum on Saturday as Iraq's minority Sunnis began the fasting month of Ramadan, which U.S. commanders said might see a rise in sectarian bloodshed.

The bomb -- most likely a car bomb, according to police -- struck near a tanker distributing kerosene for stoves in Sadr City, whose two million or more poor residents are the power base of cleric Moqtada al-Sadr and his Mehdi Army militia.

Among other violent incidents across the country, police in Tikrit said gunmen had beheaded nine people including some policemen after dragging them out of two cars in a nearby town.

Three U.S. soldiers were killed by two roadside bombs, in Baghdad and near the violent northern oil city of Kirkuk, and an American working for the State Department and a Danish soldier died in attacks around the Shi'ite southern city of Basra.

In addition to the 34 people killed in Sadr City, 35 were injured, many badly burned. Sunnis claimed responsibility, declaring the attack revenge for killings by Shi'ite militia.

"This operation comes in reaction to the crimes of the Mehdi Army against our Sunni kin in Baghdad," the Jamaat Jund al-Sahaba -- Soldiers of the Prophet's Companions -- said in a claim posted on the Internet. "Our swords can reach the depth of your areas, so stop killing unarmed Sunnis."

U.S. commanders had warned for weeks that they expected a surge of violence to accompany the holy lunar month, having observed similar patterns in previous years.

Shi'ites and Sunnis have separate systems for declaring Ramadan's start; most Sunnis began observing it on Saturday and most Iraqi Shi'ites are expected to begin on Monday.

The Americans say they are determined to end sectarian killings in Baghdad, which have soared since an attack on a Shi'ite shrine in Samarra in February. They have put in place a system of checkpoints and some fortifications around its perimeter, expected to be ready in the next few days.

CRACKDOWN

In a six-week-old security crackdown in the capital, a division of 15,000 American soldiers has secured scattered neighborhoods and is due to move into more. They say killings are fewer in the zones they have targeted.

But though data are scant, violence in the city as a whole has seemed, if anything, to worsen over the past two weeks. Hundreds of bodies have been found on the streets showing signs of torture and attacks on U.S. troops have been more frequent.

A senior U.S. military official said the American force in Baghdad is not big enough to secure the city on its own.

"That would take a lot more forces to do that," he said. "That would take Iraqi security forces."

The U.S. commander of the Baghdad operation, Major General James Thurman, said on Friday the Iraqi government had failed to deliver some 3,000 men he had asked for as reinforcements.

The Iraqi Army said it seized a leader of al Qaeda-allied militant group Ansar al-Sunna. Brigadier Qasim al-Moussawi told Reuters that Muntasir al-Jibouri -- held with two others overnight in a village near Muqdadiya, 80 km (50 miles) northeast of Baghdad -- was Ansar's leader in Diyala province.

The group denied any leader was arrested and, in an Internet posting, claimed the killing of 10 South Asian Shi'ites in Iraq.

Other Sunni militants also distributed violent videos. One, aired on the Arabiya television network, was apparently the first to show the new leader of al Qaeda in Iraq, Abu Ayub al- Masri also known as Abu Hamza al-Muhajir, since he replaced Abu Mussab al Zarqawi, killed by U.S. forces in June.

The station said Masri shot dead a Turkish prisoner on the tape, but did not broadcast the killing or any sound.

Another video, which appeared on the Internet, purported to show the bodies of two U.S. soldiers being dragged behind a truck and burned, after they were captured near Baghdad in June.

A spokesman for the Accordance Front party said its leaders met Shi'ite Prime Minister Nuri al-Maliki to urge protection for Sunnis after gunmen attacked a Sunni area in Baghdad on Friday.

Maliki has vowed to use his new security forces to curb militias but it is unclear if he can make good on his promise.