(Adds U.S. embassy)

By Mariam Karouny and David Clarke

BAGHDAD, Oct 9 (Reuters) - Two suicide car bombs killed 22 people in northern Iraq on Tuesday in attacks targeting a police chief and a tribal leader working with U.S. forces, part of an upsurge in violence that killed 56 across the country.

The spate of attacks across Iraq, which also wounded nearly 120 people, marked one of the bloodiest days during the holy Muslim fasting month of Ramadan.

Al Qaeda in Iraq has vowed to target officials and Sunni Arab tribal leaders who have joined the U.S. military to combat the Sunni Islamist group, pledging to ramp up attacks in Ramadan, which is expected to finish this weekend.

In Baghdad, guards escorting a convoy of vehicles opened fire on a car in the central district of Karrada, killing two women, the government said. A Dubai-based security company, Unity Resources Group, said one of its teams was involved in a shooting in Karrada but gave no details on casualties.

It was the second time a private contractor has been accused of killing Iraqi civilians in less than a month.

Anger at private security contractors is high in Iraq after a Sept. 16 shooting involving U.S. firm Blackwater in which 17 people were killed. The latest shooting will likely heighten Iraqi calls for tighter controls over the scores of security firms in Iraq, which are immune from local law.

Government spokesman Ali al-Dabbagh said Tuesday's shooting in Karrada was unprovoked.

"There has been an incident, an attack on civilians. Two Iraqi women were killed," Dabbagh said.

One witness said the guards fired a warning shot when a car carrying two women and children pulled out of a side road. But the driver edged forward and the security guards opened fire.

Unity Resources Group said it regretted the incident involving its security team.

"The first information that we have is that our security team was approached at speed by a vehicle which failed to stop despite an escalation of warnings which included hand signals and a signal flare," the firm said in a statement.

"Finally shots were fired at the vehicle and it stopped."

U.S. embassy spokeswoman Mirembe Nantongo said a shooting had occurred involving a private security firm employed by an American non-profit organisation under contract to USAID. She did not name the organisation.

DEVASTATION IN BAIJI

In the northern town of Baiji, officials said the police chief was wounded and the condition of the Sunni Arab tribal leader was unknown after the two suicide car bombings.

"Look at this. Is this acceptable? Does God accept this?" said a youth holding torn, blood-splattered pages of the Koran outside a mosque hit by one of the blasts in Baiji.

Baiji, 180 km (110 miles) north of the capital in Salahuddin province, is a major oil refining centre fed with crude oil and gas from the vast fields under the nearby city of Kirkuk.

Outside the mosque, men searched through mounds of bricks for survivors. Several houses nearby were flattened and two mechanical diggers shifted rubble, while a crane hoisted huge concrete blocks into the air.

Police said the other bomb was in a pick-up truck aimed at Baiji's police chief, Colonel Saad Nifous, who was wounded in the blast. Police and the U.S. military both said the bomb by the mosque had targeted a Sunni Arab tribal leader.

Police did not immediately have a breakdown of the 22 dead from the two separate attacks.

"Without question this is al Qaeda in Iraq. It is a known tactic and we have seen this time and time again," said Lieutenant-Colonel Michael Donnelly, spokesman for U.S. troops in northern Iraq.

There was confusion about which tribal leader was targeted but both those mentioned by different police sources were senior members of Sunni Arab "Awakening" councils in the area.

The councils are based on a model first used in western Anbar province, where Sunni Arab sheikhs joined with U.S. forces to drive al Qaeda militants from much of the vast desert region.

Anbar was once the heart of the Sunni Arab insurgency and the most dangerous region for U.S. troops. It has become safer since tribal leaders organised young men into police units.

The military blames al Qaeda in Iraq for most mass-casualty attacks in the country. The group also often claims responsibility for killing officials and tribal leaders.

(Additional reporting by Aseel Kami, Mussab Al-Khairalla and Wissam Mohammed)

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