"The secretary general is part of the investigation, is a subject like anyone else involved in oil-for-food at the secretariat," said the UN spokesman, Fred Eckhard.

He originally said two interviews had taken place last year in Mr Annan's office at the UN headquarters but later updated the information, saying the latest round had been yesterday afternoon and lasted an hour and 35 minutes. Previous interviews were conducted on November 9 and December 3.

Investigators, led by Paul Volcker, are examining accusations that administrators in the oil-for-food programme took bribes and allowed Saddam Hussein to skim money from the programme. Mr Volcker had been expected to release a preliminary report in late January, but he said yesterday it would come out in early February.

"All can I can tell you is wait for the report to come out." He said this month his investigation had not turned up a smoking gun.

Mr Annan, who appointed Mr Volcker, has found himself in a vulnerable position. His critical remarks about the US invasion of Iraq the backing of the UN security council has prompted the White House to lobby for his replacement.

Launched in December 1996, the oil-for-food programme allowed Saddam's regime to sell unlimited quantities of oil provided the money went primarily to buy food, medicine and humanitarian goods for Iraqis and pay reparations to victims of the 1991 Gulf war.

A report in October by the US arms inspector Charles Duelfer said Saddam had been able to "subvert" the $60bn programme to generate an estimated $1.7bn in revenue outside UN control from 1997 to 2003. Saddam also raked in more than $8bn from illicit oil deals with Jordan, Syria, Turkey and Egypt, according to US congressional investigators.

The report alleged that Saddam had issued secret vouchers for the purchase of Iraqi oil to UN officials and an array of officials and political figures from various countries - especially France, Russia and China - reportedly to curry favour with security council members. That oil could then be resold at a profit.

There are at least five US congressional investigations into the scandal, which has been a blow to the UN and has led some to call for Mr Annan's resignation.