Some peacekeepers are said to have provided fighters with guns
The head of United Nations operations in DR Congo has defended the UN against charges of a cover-up.
The BBC has learned that the UN ignored or suppressed evidence that its troops in DR Congo gave arms to militias, and smuggled gold and ivory.
It heard allegations that inquiries were halted for political reasons.
But Alan Doss said that he had personally never experienced any political pressure to abort or adjust the findings of any investigation.
He also said the charges were not new.
Mr Doss acknowledged that some United Nations personnel had behaved inappropriately, but said this should not mean the countries that sent them were culpable.
"I think it would be very unfair to smear whole countries and their contingents for irresponsible behaviour and sometimes illegal behaviour, I have to say, by a group of individuals or a few individuals," he said.
The allegations, based on confidential UN sources, involve Pakistani and Indian troops working as peacekeepers.
Gold and ivory
The United Nations Mission in the Democratic Republic of Congo (Monuc) is the global body's largest, with 17,000 troops spread across the country.
The BBC's Martin Plaut, who returned to DR Congo to follow up his initial investigation into the allegations, says the force has managed to bring a measure of stability since it was first established by the UN in February 2000.
It has also helped disarm the warring factions, run democratic elections and assisted with reconstruction.
But an 18-month BBC investigation for Panorama has found evidence that:
- Pakistani peacekeepers in the eastern town of Mongbwalu were involved in the illegal trade in gold with the FNI militia, providing them with weapons to guard the perimeter of the mines.
- Indian peacekeepers operating around the town of Goma had direct dealings with the militia responsible for the Rwandan genocide, now living in eastern DR Congo.
- The Indians traded gold, bought drugs from the militias and flew a UN helicopter into the Virunga National Park, where they exchanged ammunition for ivory.
Pakistani peacekeepers have helped train the army in DR Congo
Two FNI leaders known as "Kung-Fu" and "Dragon", who have been jailed in the capital, Kinshasa, have stated publicly that they received help from the UN.
The BBC managed to get into the maximum security jail and both confirmed this.
But the UN's head of peacekeeping, Jean-Marie Guehenno, said the BBC and the UN investigators had been given different stories from the same interviewees.
He said "Kung-Fu" had every interest in discrediting the troops who helped put him in jail and disrupt his criminal activities.
UN insiders close to the investigation told the BBC they had been prevented from pursuing their inquiries for political reasons. Our correspondent says that, in short, the Pakistanis, who are the largest troop contributors to the UN in the world, were too valuable to alienate.
Pakistan has said it has no means of influencing the UN and that there is no evidence that its troops were involved in an illegal gold trade or re-arming militia, and described the allegations as baseless.
The Indian Army told Panorama that an investigation by the UN watchdog had revealed that all but one of the allegations were based on hearsay or were not backed by credible evidence.
Mr Guehenno insisted he was not under pressure not to annoy the Indian and Pakistani governments, adding that the UN had done more in terms of discipline and good conduct in the last three or four years than in the previous 50 years of its existence.
"It would be a deep mistake to think that you can cover up and that it would be wise to cover up," he said. "The only capital we have is our integrity and that must be protected at all costs."
He said the BBC report had not really revealed any new information.
And the head of the United Nations division responsible for investigating corruption and mis-management, Inga Britt Ahlenius, has also defended the UN against the charges.
She said reports she had commissioned which showed a picture of considerable disarray in the organisation - and which have been seen by the BBC - refer to difficulties which are now in the past, and that new managers are in place.