Inga-Britt Ahlenius, head of the UN office of internal oversight services (OIOS), said in a press conference: "Our caseload has been very steady over the last three months, around 250 cases.
"We found mismanagement and fraud and corruption to an extent we didn't really expect."
Two-thirds of cases being reviewed related to peacekeeping missions and about 80 were investigating alleged sexual exploitation and abuse.
Ahlenius said that investigators had already discovered that fraud was present to some degree in contracts worth about $600 million.
So far 25 reports on mismanagement, fraud and corruption have been submitted to the UN's management by OIOS and its procurement task force.
Rooting out fraud
Robert Appleton, head of the procurement task force, a temporary body established in 2006 after corruption was exposed in the UN oil for food programme in Iraq, said the the number of irregular contracts was not significant and many allegations could not be supported.
Appleton said: "There's no question that some of the large contracts here have been tainted, but in terms of the number of contracts, it's not anywhere near the majority."
Ban Ki-moon, the UN secretary-general, said on Monday that the international body needed greater investigative powers to probe its own activities and root out fraud.
This comes after Ban was criticised for awarding a single-source contract worth $250 million to a unit of US defence firm Lockheed Martin Corp without competitive bidding.
Ban said he was permitted to award such contracts in exceptional cases where the the supplier could deliver in short notice.