UNITED NATIONS — A U.N. task force has uncovered a pervasive pattern of corruption and mismanagement involving hundreds of millions of dollars in contracts for fuel, food, construction, and other materials used by U.N. peacekeeping operations, which are in the midst of their largest expansion in 15 years.

In recent weeks, 10 procurement officials have been charged with misconduct for soliciting bribes and rigging bids in Congo and Haiti. It has been the largest single crackdown on U.N. staff malfeasance in the field in more than a decade.

The task force has issued a series of public and confidential reports charging that corruption has spread from U.N. headquarters — where three officials have been convicted in bribery schemes — to the far reaches of its growing peacekeeping empire. The task force has also cast a spotlight on the U.N.’s repeated failure to take action against officials suspected of wrongdoing, allowing them to carry out criminal schemes in one U.N. mission after another.

“The task force identified multiple instances of fraud, corruption, waste and mismanagement at U.N. headquarters and peacekeeping missions, including ten significant instances of fraud and corruption with aggregate value in excess of $610 million,” said one report by the task force, headed by a former federal prosecutor in Connecticut, Robert Appleton. The crop of new corruption cases highlights the limits of reforms imposed since the early 1990s, when a previous buildup of peacekeeping missions led to reports of rampant corruption in Cambodia, Somalia, and the Balkans. In response, the United Nations created the Office of Internal Oversight Services in 1994, but it has a poor record of holding corrupt officials to account.

The recent investigation in Congo uncovered “widespread and inherent corruption” throughout the mission’s purchasing department. One official targeted in the probe, Abdul Karim Masri, had emerged unscathed from repeated OIOS inquiries into his alleged activities over more than a decade. The task force charged that Mr. Masri, 54, engaged in an “extensive pattern of bribery” during his seven years in Congo, according to a confidential account of the probe.

The unit claimed Mr. Masri accepted a $10,000 bribe from a boating company, steered a lucrative catering contract to a friend, and persuaded one U.N. contractor to paint his apartment and swimming pool for free and another to supply him with a steep discount on a Mercedes-Benz. It also described an effort by Mr. Masri to solicit a kickback from a construction executive on a $5.5 million contract to refurbish an airfield in eastern Congo.

“We are the ones deciding the case,” Mr. Masri told him, according to the account of a meeting at Mr. Masri’s Kinshasa home. “It’s in our hands.”

Mr. Masri declined repeated requests by e-mail to comment on the findings, saying that U.N. rules do not permit him to “deal with the press.” He referred questions to U.N. staff counsel Edwin Nhliziyo, a retired U.N. auditor who served with Mr. Masri in Congo. “He has denied all these allegations,” Mr. Nhliziyo said. “If the U.N. has the evidence to back all of this stuff up, that is fine. At this point it’s just allegations, and he is innocent until proven guilty.”

In Haiti, the United Nations charged five employees with misconduct after the task force established that they had steered a $10 million-a-year fuel contract to a Haitian company, Distributeurs Nationaux S.A., according to U.N. officials and confidential documents. The task force has been unable to prove that the five profited from the scheme, citing its lack of authority to subpoena bank records, but they recommended the case be referred for criminal prosecution by authorities in Haiti or America.

U.N. officials privately contend that some malfeasance is inevitable in an organization that processes 12,000 purchase orders in the field each year and buys enough energy to power a city larger than Washington, D.C. They also noted that some previous U.N. corruption crackdowns have unraveled under closer examination.

“I don’t believe there is, or that [the investigators] have found, a culture out there of fraudulent behaviors,” an Australian national who administers U.N. peacekeeping missions, Philip Cooper, said. “We have our share of malpractice, but we do not have many cases of straight-out fraud like has been found in the Congo.”

The latest investigations grew out of the probe into the U.N.’s Iraqi oil-for-food program by a former Federal Reserve Board chairman, Paul Volcker. It comes as spending on peacekeeping operations is rising — from $2.2 billion in 2004 to $7 billion — supporting a force of more than 100,000 peacekeepers.

Mr. Volcker’s team helped uncover a bribery scheme by a U.N. procurement officer, Alexander Yakovlev of Russia, who pleaded guilty in August 2005 to federal charges that he received nearly $1 million in kickbacks for steering contracts to favored companies.