A mystery plane crash which killed former United Nations (UN) secretary-general in 1961, may have been caused by a deliberate “external attack”, a new investigation has concluded.

Dag Hammarskjold was flying on a controversial peace mission to the war-torn Congo when his DC-6 aircraft went down in modern day Zambia despite perfect conditions and an experienced crew.

The UK, US, South Africa and Russia – who all had official or unofficial forces involved in the civil conflict – are accused of withholding information about the tragedy in the UN’s new 95-page report into the tragedy

All four countries, along with a host of other western powers, had a vested interest in the war as major international companies sought to gain access to the region’s mineral wealth following the then DR Congo's independence from Belgium a year earlier.

It was into this febrile country – today, the Democratic Republic of Congo – that Mr Hammarskjold, a Swedish national, was flying.

“It appears plausible that an external attack or threat may have been a cause of the crash, whether by way of a direct attack…or by causing a momentary distraction of the pilots,” the new report by Mohamed Chande Othman, a former chief justice in Tanzania, states.

The investigation builds on new information, Mr Othman said, including new explanations about possible causes, crash site photos, intercepts of relevant communications, the role of air traffic control and further information on the military capacities of different parties in the region.

“The presence in the area of foreign paramilitary, including pilots, and intelligence personnel,” meant an attack could not be ruled out, the report states.

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But while the investigation was said to have made progress, Antonio Guterres, the current UN secretary-general, said it was “clear that the work will need to continue with renewed urgency, with a view to establishing the truth of the tragic event”.

He said that 14 countries had been asked to appoint an independent person to perform a comprehensive internal review of intelligence, security and defence archives as part of the investigation.

Most had complied but the UK, US, South Africa and Russia had all failed to provide “substantive” responses to queries about key documents and possible evidence they may hold, he said.