Blackwater, the US private security firm threatened with expulsion from Iraq, has resorted to armed force twice as often as any other other mercenary company in the country, according to Pentagon officials.

The North Carolina outfit is the main contractor used to provide bodyguards and escorts for US State Department officials.

Its employees stand accused of firing indiscriminately into civilian traffic on September 16 in the Mansoor district of Baghdad, killing 11 civilians and wounding another 15.

The US military has now sent a team of five investigators to Iraq to report on this and six other incidents in which 10 more civilians died.

All mercenary companies on government contracts were exempted from local legal jurisdiction by the Coalition Provisional Authority, the body which ran Iraq until a new government could be elected after Saddam's fall from power in 2003.

Blackwater, which has earned a reputation among the 48,000 mercenaries operating there for shooting first and asking questions afterwards, has clashed repeatedly with an Iraqi administration which accuses it of indiscriminate use of lethal force.

Its employees - mainly former US special forces' personnel - have opened fire twice as often as those of DynCorp and Triple Canopy, the other two firms under US State Department contracts.

Blackwater has its own fleet of light helicopter gunships and mine-protected vehicles and employs about 850 people in Iraq. The firm has earned almost £500m in government contracts since 2002. But the company's aggression is now seen by the US military as counter-productive to its own "hearts and minds" campaign.

Congressman Jan Schakowsky, an Illinois Democrat, said yesterday: "It's really affecting attitudes toward the US when you have these cowboys out there. These guys represent the US to the Iraqi population and there are no rules of the game for them."

No private security employee has yet been prosecuted because of the legal muddle over authority to bring them to account for their actions.

Blackwater refuses to answer any questions on the conduct of its employees.

Meanwhile, Iraq's top Shi'ite cleric Grand Ayatollah Ali al Sistani met the country's Sunni vice president yesterday for the first time to discuss a new initiative aimed at uniting feuding politicians.

Deep sectarian rifts in Iraq have hampered progress on key laws that Washington wants passed to help reconciliation between majority Shia and minority Sunni Arabs.

Vice President Tareq al Hashemi, who heads the Sunni Islamic Party met the reclusive Sistani in Najaf.

Hashemi said the purpose of the meeting had been to discuss the new initiative, the Iraqi National Compact.

"The meeting was profound and many issues related to the political process were discussed," Hashemi said.

The Iraqi National Compact is a set of 25 political principles unveiled by Hashemi's party on Wednesday aimed at removing deep mistrust among politicians.