WASHINGTON (REUTERS) - The State Department said on Tuesday it was investigating allegations of sexual misconduct by overseas staff including a report that an ambassador patronised prostitutes after a leaked memo said the agency had ignored the misbehaviour.

CBS News this week reported it had obtained an internal State Department inspector general's memo that said several investigations into possible cases of misconduct were influenced, manipulated or called off.

The memo, according to CBS, listed eight examples of alleged misconduct. They include allegations that a State Department security official in Beirut "engaged in sexual assaults" on foreign nationals hired as embassy guards, and that members of former Secretary of State Hillary Clinton's security detail "engaged prostitutes while on official trips in foreign countries" - a problem the report says was "endemic".

The memo, CBS said, alleges an underground drug ring operating near the US Embassy in Baghdad, which supplied State Department contractors with drugs. It said a US ambassador in a sensitive diplomatic post was suspected of "patronising prostitutes in a public park", according to CBS.

Although the CBS report did not identify employees alleged to have been involved, US Ambassador to Belgium Howard Gutman issued a statement on Tuesday denying he did anything wrong.

"I am angered and saddened by the baseless allegations that have appeared in the press and to watch the four years I have proudly served in Belgium smeared is devastating," Mr Gutman said.

"I live on a beautiful park in Brussels that you walk through to get to many locations and at no point have I ever engaged in any improper activity."

A spokesman for the State Department's Office of Inspector General said on Tuesday the allegations were being investigated and former officials with law enforcement experience were hired to help with the investigation.

State Department spokesman Jen Psaki told reporters the memo included "a number of unsubstantiated accusations".

"Prior to the drafting of this memo, the Department's Bureau of Diplomatic Security had already started looking into or completed the process of looking into these individual incidents," Ms Psaki said.

"All of these cases have been and will be brought... to their logical conclusions." she said, "The notion that we would not vigorously pursue criminal misconduct is not only preposterous, it's inaccurate."

The White House vowed no tolerance for misconduct.

"We're not going to prejudge anyone or anything before all the facts are determined," White House spokesman Jay Carney said at his daily briefing. "I want to make clear the president has zero tolerance for misconduct by any government employee."

Internal State Department investigations are conducted by the Diplomatic Security service, which protects Secretary of State John Kerry and ambassadors. An inspector general's report in February found shortcomings in the Diplomatic Security service's ability to investigate problems, a problem currently under review by the State Department.

The inspector general spokesman also said cases in which criminal behavior was found would be referred to the Justice Department. Staff found to have broken internal rules would be subject to administrative disciplinary action, he added.

In a letter to Mr Kerry, Representative Ed Royce, a Republican who chairs the House Foreign Affairs committee, said on Tuesday he was "deeply troubled" by the allegations in the CBS report and called for a thorough investigation.