A UN investigator looking into new evidence on the death of the UN secretary general Dag Hammarskjöld in a 1961 plane crash has criticised the UK and South Africa for their lack of cooperation.

In a progress report on his investigation, Mohamed Chande Othman said fresh information had surfaced in the past year that could cast light on the crash in what was then Northern Rhodesia on the night of 17 September 1961, when Hammarskjöld was on his way to talks aimed at ending civil war in neighbouring Congo.

The UN appointed Othman, a former chief justice of Tanzania, in 2015 after new evidence raised questions over whether the secretary general’s DC-6 plane could have been shot down at a small airstrip at the town of Ndola.

At the time, local residents reported seeing a second plane in the night sky, but their testimony was downplayed or ignored by the British colonial authorities.

Othman’s inquiries have focused on aircraft being operated by separatists in Katanga province who were backed by Belgian mining interests in their fight against the newly independent Congolese government in Léopoldville (now Kinshasa) and the UN forces supporting it.

Dag Hammarskjöld: evidence suggests UN chief's plane was shot down Read more

The investigation has also focused on the role of European paramilitaries and mercenaries fighting with the separatists. One theory being explored is that Hammarskjöld’s plane was shot down by a Belgian mercenary pilot in a Fouga warplane secretly supplied to the rebels by the French government.

In his report (pdf), the judge said the UK and South Africa had been entirely unhelpful. Both governments ignored a UN general assembly resolution calling for the appointment of “an independent and high-ranking official to conduct a dedicated internal review of their intelligence, security and defence archives” for relevant information.

Othman wrote to nine member states asking for cooperation in March this year, and six of them appointed a special official as requested. Of the remaining three, Russia said it would conduct a review of its intelligence and security archives, while the UK and South Africa provided no new cooperation of any kind.

Othman only heard back from the British government last month, eight months after he wrote to request cooperation and after he had already drafted his progress report. In its response, the UK refused to appoint an official to cooperate with the inquiry, saying that it had already released everything it had on the incident.

The judge, who is due to complete his inquiry in 2019, rejected that argument, saying it was “almost certain” the UK still had undisclosed information of “probative value”.

Facebook Twitter Pinterest Dag Hammarskjöld was on his way to talks aimed at ending civil war in Congo when his plane crashed. Photograph: AP

Othman reported that South Africa had not answered his requests for assistance, despite efforts by the current UN secretary general, António Guterres, and his predecessor, Kofi Annan.

In his interim report, Othman said it was premature to draw conclusions from the lack of cooperation, but added it “may be seen as a failing in the international community’s collective and ongoing effort in the search for the truth of the tragic event”.

Susan Williams, a senior research fellow at the University of London and the author of a book on the Hammarskjöld case. “This report is of huge and troubling significance, because it reveals that the UK and South Africa have slammed the door shut,” she said.

The South African mission to the UN did not respond to a request for comment. The UK Foreign Office said it had cooperated with the investigation in the past and had nothing to add.

“Following a search across all relevant government departments in 2017, the UK provided additional information that we believed might be helpful to the inquiry. We have no further information of direct relevance to the inquiry,” an FCO spokesperson said.

Sources familiar with the UN investigation said the information supplied by the UK had been largely from Colonial and Foreign Office archives, with little or nothing from the intelligence agencies, MI6 or GCHQ.

The Othman investigation has found that British intelligence was heavily involved in the region. According to official and unofficial accounts of events at the time, an MI6 official, Neil Ritchie, had commandeered a privately owned helicopter on the eve of the crash to fly to the headquarters of the Katanga separatist leader, Moïse Tshombe, to persuade him to meet Hammarskjöld.

Tshombe agreed to fly across the border to Ndola for talks but left when the UN secretary general’s plane failed to appear. The wreckage was found the next day, 18 September, by local woodcutters.

19 September 1961: UN secretary-general Dag Hammarskjold dies in plane crash Read more

There was only one survivor among the 16 passengers and crew, the security officer Sergeant Harold Julien. He died a few days after the crash, but told officials there had been explosions just before the plane came down.

In an earlier progress report, Othman found that the US and UK governments had intercepted radio traffic in the area and suggested the 57-year mystery could be solved if transcripts of those recordings were produced.

The US has appointed an official to work with Othman’s investigation but has yet to hand over intelligence files on the crash.

“The UK states that all information of value is available, but this clearly does not apply to its security and intelligence agencies,” said Williams, whose book, Who Killed Hammarskjöld?: the UN, the Cold War and White Supremacy in Africa, helped lead to the reopening of the case.

Williams said the UK had failed to produce records of Ritchie’s and other MI6 activities along the Rhodesian-Congolese border. She also pointed out that in 1998 Archbishop Desmond Tutu handed over to South Africa’s justice minister a set of documents about a plot by a paramilitary organisation to kill Hammarskjöld.

“What happened to these documents?” she asked. “If the UK and South Africa have nothing to hide, why have they chosen not to cooperate with the UN?”