"We are taking very active measures to ensure that we do not have any more bad apples in that department," Annan said on Wednesday.
"We are going to act very energetically to take measures to ensure that what happened does not happen again."
A UN-established investigation on Monday accused Alexander Yakovlev, a veteran purchasing official, of getting about $1 million in illegal payments from the winners of UN contracts worth $79 million.
The inquiry, led by former US Federal Reserve Chairman Paul Volcker, also accused Yakovlev, a Russian, of working with a French friend to solicit a bribe from a Swiss firm bidding for a UN oil inspection contract.
Yakovlev was arrested on Monday and pleaded guilty in a federal court in New York to conspiracy, wire fraud and money laundering charges.
The charges were based on evidence gathered in an internal investigation by the UN Office of Internal Oversight Services, and UN officials said that probe was widening.
"We are taking very active measures to ensure that we do not have any more bad apples"
Annan said he planned to meet with his new undersecretary-general for management, Christopher Burnham, later in the week to discuss new procurement safeguards.
Burnham previously served as the top management official at the US State Department. He was brought in by Annan in May to oversee UN administrative reforms.
The Volcker committee was set up to investigate alleged corruption and mismanagement in the $64 billion oil-for-food programme, the biggest UN humanitarian aid program before it was shut down in 2003 following the US led invasion of Iraq.
The Yakovlev case was a strong sign that corruption at the United Nations extended beyond the oil-for-food programme.
The Volcker panel on Monday also accused Benon Sevan, the head of the oil-for-food plan from 1997 to 2003, of receiving nearly $150,000 in kickbacks for steering oil allocations to a Swiss-based trading firm.
Sevan has strongly denied any wrongdoing and is now in his native Cyprus, which has no extradition agreement with the