The diplomats, who spoke on condition of anonymity, said insurgents are being airlifted from the southern province of Helmand to the north amid increasing violence in the northern parts of the country.
The aircraft used for the transfer have been identified as British Chinook helicopters.
The officials said Sultan Munadi, an Afghan interpreter who was kidnapped along with his employer, New York Times reporter Stephen Farrell, was killed by a “British sniper” as commandos executed a rescue operation to free Farrell.
They said Munadi was targeted for possessing documents and pictures pointing at the British military's involvement in the transfer operation.
The Afghan journalist also had evidence of the involvement of the foreign forces in Afghanistan in the tensions that rocked China's Xinjiang autonomous region in July, the diplomats said.
American forces have also invigorated the insurgency in the war-ravaged country by outfitting the Taliban with Russian-made weaponry used during the 1979-89 Soviet invasion of Afghanistan, which was fought against by the Afghan Mujahedeen, the diplomats said.
The US forces are assumed to have gathered the armaments during a campaign to "collect weapons from irresponsible people," after the 2001 invasion.
Diplomats said Afghan Interior Minister Mohammad Hanif Atmar, a Pashtun who has received his higher education in the UK, was still operating under the British guidance.
The Interior Ministry is accused of enabling the provision of arms and ammunition for the north-based militants by the Pashtun police force.
Earlier in the week, Afghan President Hamid Karzai was quoted by the BBC Persian as having ordered an investigation into reports of 'unknown' army helicopters carrying gunmen to the north.
The Afghan president said based on unconfirmed reports, the helicopters have been taking gunmen to Baghlan, Kunduz and Samangan provinces overnight for about five months now.
In early 2008, Karzai expelled two British diplomats for allegedly planning to “turn” senior Taliban commanders. According to the Times Online, the British officials had sought to persuade militant chief Mullah Mansoor Dadullah to cooperate with the UK.
Afghanistan is currently witnessing the highest level of violence since the invasion, despite the presence of more than 100,000 foreign troops.