Erik Prince's company has been paid more than $1bn for its services

Blackwater has come under scrutiny since a shoot-out last month in Baghdad in which 11 Iraqis were killed. The FBI has begun investigating that incident.

The firm's founder, Erik Prince, said his staff were brave and effective, and had acted "appropriately".

But one of the committee asked if the firm was "a shadow mercenary force".

"Blackwater appears to have fostered a culture of shoot first - and sometimes kill - and then ask the questions," said Democratic Congressman Elijah Cummings.

Blackwater is the main firm employed by the state department to provide security for its staff and visiting officials and businessmen.

It has earned more than $1bn (£490m) from US government contracts since 2001. The state department paid the company more than $832m (£408m) for security work between 2004 and 2006.

'Short' on standards

The 16 September shooting incident is not featuring in the hearing by the Committee on Oversight and Government Reform, following a Justice Department request that it wait until the FBI's investigation is concluded.

CONGRESS BLACKWATER REPORT Earned more than $1bn (£490m) from US government contracts since 2001 Staff involved in 195 shootings in Iraq between 2005 and 12 September 2007 Allegedly drunk employee killed Iraqi vice-president's guard in 2006, but was flown out of Iraq and faced no criminal charges 28 staff in Iraq sacked for weapons-related incidents, 25 for alcohol and drug violations

Committee on Oversight and Government Reform Blackwater memorandum

But congressmen raised a number of other incidents they said showed that Blackwater fell short of the standards of the US military.

The committee's chairman, Henry Waxman, referred to an incident in Afghanistan in 2004 when a Blackwater plane flown by inexperienced pilots crashed, killing US service personnel on board.

An investigation showed the pilots did not follow procedure, did not know where they were going and were treating their mission as "fun".

"Is Blackwater, a private military contractor, helping or hurting our efforts?" Mr Waxman asked, referring to Iraq.

'Unaccountable'

Several representatives referred to an incident in which a Blackwater guard shot dead a bodyguard of one of Iraq's vice-presidents while drunk. He was hurried out of Iraq and faced no criminal charges.

They said the incident showed how "unaccountable" Blackwater and other firms were.

"Why are we privatising our military to an organisation that has been aggressive and in some cases reckless in the handling of their duties?" asked New York Democrat Carolyn Maloney.

BLACKWATER USA FACTS Founded in 1997 by a former US Navy Seal Headquarters in North Carolina One of at least 28 private security companies in Iraq Employs 744 US citizens, 231 third-country nationals, and 12 Iraqis to protect US state department in Iraq Provided protection for former CPA head Paul Bremer Four employees killed by mob in Falluja in March 2004

Profile: Blackwater USA Security firms in the dock

Mr Prince said Blackwater had taken firm action against the guard. He was fined and fired.

"But we, as a private organisation, can't do any more," he said. "We can't flog him. We can't incarcerate him."

He also said that 30 Blackwater staff had been killed while working to defend US diplomats, but that none of its clients had died while in its protection.

"There is no better evidence of the skill and dedication of these men," said Mr Prince, who is a former member of the US Navy Seals special forces.

Mr Prince, 38, said his firm was happy to be subjected to greater oversight.

"Blackwater believes that more can and should be done to increase accountability, oversight and transparency," he told the hearing.

Good value?

Mr Waxman's staff produced a scathing report on Monday that released details of several incidents involving Iraqi casualties, in which Blackwater employees had fired first on 163 out of 195 occasions.

In the majority of cases, the guards fired their weapons from moving vehicles without stopping to count the dead or assist the wounded, the report said.

He questioned whether private contractors should be used instead of US soldiers.

"The question for this hearing is whether outsourcing to Blackwater is a good deal to the American taxpayer," he said.