Christopher Burnham, the UN undersecretary-general for management, said "the potential abuse could go into the tens of millions of dollars", adding "it could go higher than that but we are in the middle of looking at 200 different reports of abuse".
The findings threaten to further taint the world body's reputation following investigations that turned up widespread corruption and mismanagement in the now-defunct UN oil-for-food programme in Iraq.
John Bolton, the US Ambassador said, "it is very disturbing. It shows the sad record of mismanagement that we are trying to deal with through the UN reform process".
A just-completed audit by the Office of Internal Oversight Services (OIOS), the UN internal watchdog agency, "raises a number of serious allegations and concerns" in peacekeeping procurement, Burnham told reporters.
Following up on that audit, the OIOS was now adding staff and searching for criminal wrongdoing in peacekeeping contracts going back five years, he said. It would turn over any evidence of illegality to US law enforcement authorities, he added.
"The potential abuse could go into the tens of millions of dollars...it could go higher than that"
UN undersecretary-general for management
The peacekeeping department fields some 85,000 troops, police and civilians in missions around the world. Peacekeeping makes up about 85% of total UN procurement spending.
Burnham's comments came a week after the world body's two most senior procurement officials were indefinitely suspended from their duties with pay, along with six other staff members, while investigators carried on their search for improprieties.
Burnham stressed the eight suspensions were administrative rather than disciplinary actions and did not imply wrongdoing.
Both of the suspended senior procurement officials - Andrew Toh, who heads the UN Office of Central Support Services, and Procurement Division chief Christian Saunders, who reports to Toh - have denied any impropriety.
The OIOS audit, which has not been made public, reported "substantial evidence of abuse in procurement for peacekeeping operations, leading to financial losses and significant inaccuracies in planning assumptions", Burnham said.
"The design and maintenance of the controls needed to ensure that the UN procurement complied with financial rules and regulations were insufficient," he said, reading from the audit report. "Important controls were lacking while existing ones were often bypassed."