Investigators have called on the UN to reopen an inquiry into the 1961 death of Dag Hammarskjold, the then UN chief, citing "persuasive evidence" that his aircraft was shot down.

The inquiry on Monday called on the US National Security Agency (NSA) to release cockpit recordings from the time to confirm whether a mercenary fighter jet may have shot down the aircraft.

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 it was scheduled to land at the 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 prominent international judges and diplomats.

Sole survivor

Fifteen people including Hammarskjold died when the DC-6, known as the Albertina, crashed into the ground near Ndola as it came in to land in advance of a meeting between him and Moise Tshombe, the Katangan leader.

A sole survivor of the crash died just 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 said.

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.

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.

'Greatest statesman'

The commission called on the UN General Assembly to take the investigation further, starting with obtaining the cockpit recordings from the NSA.

Ban Ki-moon, the current UN secretary-general, thanked the commission, his spokesman Martin Nesirky said in a statement.

"The UN secretariat will closely study the findings of the commission's report," Nesirky said.

After Hammarskjold's death, he was eulogised by then US President John Kennedy as "the greatest statesman of our century".

But Hammarskjold was far from universally popular and Nikita Khrushchev, Kennedy's Soviet counterpart, had called for his resignation over his handling of the Katanga conflict.

Hammarskjold responded: "It is very easy to bow to the wish of a big power, it is another matter to resist."

Hammarskjold, who was 56 at the time of his death, is the only UN chief to have died while in office and also the only person to have been posthumously awarded the Nobel Peace Prize.