THE HAGUE (AFP) - Investigators on Monday called on the United Nations to reopen a probe into the 1961 death of UN secretary general Dag Hammarskjold, citing "persuasive evidence" that his plane was shot down.

The enquiry called on the US National Security Agency to release cockpit recordings from the time to confirm whether a mercenary fighter jet may have shot down the plane.

Mr Hammarskjold, the UN's second secretary-general, died in mysterious circumstances in September 1961 while on a peace mission to the newly independent Congo, when his plane crashed shortly before landing at Ndola airport in Zambia (then Northern Rhodesia).

The mineral-rich province of Katanga was at the time fighting to secede from Congo, with the backing of the West and their commercial interests in the region.

"There is persuasive evidence that the aircraft was subjected to some form of attack or threat as it circled to land at Ndola," said the 61-page report released in The Hague by a privately appointed commission consisting of high-profile international judges and diplomats.

Fifteen people including Mr Hammarskjold died when the DC-6, known as the Albertina, smashed into the ground near Ndola as it came in to land ahead of a meeting between the UN's top official and Katangan leader Moise Tshombe.

A sole survivor of the crash died days later.

"We ... consider the possibility that the plane was in fact forced into its descent by some form of hostile action is supported by sufficient evidence to merit further enquiry," the report added.

The commission cited new witnesses who claimed to have seen a second aircraft shooting at the Albertina on the night of the crash almost 52 years ago.

Several witnesses, interviewed by two commission members in May, told how they saw two planes in the sky over Ndola, the larger one on fire.

"Our answer to the question posed to us, whether the UN would now be justified in reopening an enquiry in the light of the evidence that is available, (is) we say a qualified but firm 'yes'," retired British judge Sir Stephen Sedley, the commission's chairman, told a press conference.

Mr Sedley said however that in order to further the investigation it was necessary to listen to cockpit or radio conversations believed to have been recorded by the US National Security Agency's intelligence service in 1961.

"Authenticated recordings of any such cockpit narrative or radio messages would furnish potentially conclusive evidence of what happened to the DC-6," he said.

However, until now, the commission has run into a stone wall at the NSA, being told that because they were classified as "top secret", two of three documents requested "appeared to be exempt from disclosure".

"An appeal against the continued classification of these documents, which the Commission understands to be subject to a qualified 50-year rule, has been lodged," the report noted.

The commission called on the UN General Assembly to take the investigation further, saying it "neither recommended or anticipated the resumption of the UN enquiry at large". Rather, it would be a "focused and staged resumption", starting with obtaining the cockpit recordings from the NSA before moving forward.