(Updates with U.S. comment)

UNITED NATIONS, Nov 1 (Reuters) - Following a shooting in Iraq involving U.S. firm Blackwater, U.N. Secretary-General Ban Ki-moon called on Thursday for private security companies operating in war zones to be held more accountable.

Private U.S. security firms have come under scrutiny since a shooting last month in which Blackwater security guards working for the State Department were accused of killing 17 Iraqis in Baghdad.

Ban's report to the U.N. Security Council on the protection of civilians in armed conflict did not mention the Blackwater incident. It said that as of March 2006, there were an estimated 181 private military security companies in Iraq with 48,000 employees involved in activities that frequently bring them into direct contact with civilians.

"It is well established that employees (of private security companies) must comply with international humanitarian law," Ban said, adding that the responsibilities of the states that hire them were also well established.

"However, far more needs to be done to promote compliance with the law and accountability of employees and hiring states for any violations thereof," he said.

U.S. officials said this week the Pentagon and U.S. State Department have agreed to tighten rules governing private security contractors in Iraq, giving a greater oversight role to the U.S. military.

The killing of the Iraqis created tensions between the Iraqi government and Washington, and the Iraqi government took its own steps on Tuesday to tighten controls on contractors.

It approved a draft law that would scrap a decree issued by the U.S.-led Coalition Provisional Authority in 2004, before it handed over control to Iraqis, which granted foreign contractors immunity from prosecution in Iraq.

Richard Grenell, spokesman for the U.S. mission to the United Nations, said "We take this issue very seriously and are saddened by every innocent civilian life lost. However, there have been many American lives given in trying to give the Iraqi people democracy and freedom."

Blackwater says its guards acted lawfully after being shot at, but the Iraqi government says the guards "deliberately killed" the 17 people.

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