Kofi Annan told The Independent that he was "deeply shocked and outraged" by the Congo scandal, which has led to the investigation of 150 allegations against some 50 soldiers of sexual exploitation of minors, including gang rapes.
At least seven cases of sexual exploitation and abuse have been substantiated against peacekeepers serving in Bunia, in north-eastern Congo, where the UN is helping the country recover from years of conflict. Children as young as 12 have been bribed with eggs, milk or a few dollars in exchange for sex, according to UN reports.
Although the new measures apply only to the Congo mission, where 16,000 soldiers and civilian staff are stationed, Mr Annan said: "I want to use this to send a message to all the 17 UN peacekeeping missions around the world."
The revelations of the continuing sex abuse scandal, which was first reported by The Independent last year, coupled with allegations against Mr Annan's son Kojo in the Iraqi oil-for-food scandal, have greatly damaged the UN secretary general's personal standing.
Tony Blair sprang to Mr Annan's defence at a meeting in London on UN reform yesterday, issuing a ringing endorsement of the Ghanaian UN chief who sat beside him, to applause from delegates.
Mr Annan has been under attack from right-wing critics in the United States for his stewardship of the UN's humanitarian oil-for-food programme, which is the subject of five separate investigations at the UN and in the US Congress.
On Monday, he suspended the head of the $64bn (pounds 34bn) oil-for-food programme, Benon Sevan, and another senior UN official who dealt with contracts after an interim report by former Federal Reserve Board chairman Paul Volcker accused them of misconduct. Mr Annan has rejected calls for his own resignation pending the publication of a separate Volcker report into his son's links with a company involved in monitoring the oil-for-food contracts.
But the sex abuse scandal in Congo has also contributed to the picture of UN mismanagement, which according to one Western diplomat prompted Mr Annan to declare that the UN is observing a "zero-tolerance" stand at this time.
He was asked last month by a US television reporter whether the Congo scandal is "the UN's Abu Ghraib", comparing it to the prisoner abuse by American soldiers in Iraq that prompted the US Defence Secretary Donald Rumsfeld to offer his resignation.
"Annan is having a tough time, especially in Washington, where his grip and management is being questioned," said a Western diplomat. "The reporting of misconduct by people wearing blue helmets feeds that agenda. It is important that Annan shows that he is on top of things."
Mr Annan disclosed in a letter to the Security Council that the head of the Congo mission, US diplomat William Swing, had brought in a series of measures to tackle the sexual abuse problem. These include "a strict non-fraternisation policy", a curfew for military contingents, and an awareness campaign for all staff at the mission.
The UN mission spokesman, Kemal Saiki, confirmed that "non-fraternisation" meant no sex with the local population.
Mr Saiki admitted that the scandal was having an impact on the mission's operations in the region, "and we don't want to give the impression we are brushing it under the carpet". UN regulations strictly prohibit soldiers having sex with anyone under 18 years of age and forced prostitution.
But Mr Saiki stressed that while the UN peacekeepers were supposed to conform to UN standards while serving at the mission, individual countries were responsible for disciplinary measures against offenders.
France has jailed a UN civilian staff member for rape. Allegations have also been made against soldiers from Uruguay, Morocco, Tunisia and Nepal.
"We shall get to the bottom of this to ensure that it is not repeated," Mr Annan said.
He vowed that the "entire chain of command" would be held accountable for enforcing the "zero-tolerance" standard.
An assistant UN secretary-general, Angela Kane, has been dispatched to the Congo mission: she has so far completed nine investigations, and has 10 ongoing.
In his letter to the Security Council, Mr Annan recognised the damage to the UN from the sexual abuse scandal, saying: "We cannot tolerate even one instance of a United Nations peacekeeper victimising the most vulnerable among us."