By Peta Thornycroft and Sebastien Berger
Aerial pictures show construction work is well under way, with a newly built control tower apparently complete and the runway nearly ready for surfacing.
There are other airfields within a short distance of the mining area, and no obvious need for a runway long enough for transport planes to take off and land even closer to the mines. A Western diplomat said the existence of the runway, out of sight except from the air, was "extremely" worrying.
The images also show what appears to be a tented army camp in the diamond fields, which would be in violation of Zimbabwean court orders and of an undertaking to the Kimberley Process, which was set up to prevent "blood diamonds" from conflict zones entering the global gem trade.
According to human rights groups, hundreds of independent miners were killed when soldiers seized control of the Chiadzwa area in November 2008, since when others have been compelled to work for only a fraction of the value of the diamonds they unearth. Officers use the proceeds from their sale to enhance their meagre pay - a ploy encouraged by Mr Mugabe's henchmen to help ensure the army's continued loyalty.
But the construction of the runway suggests that the army has now widened its ambition and wants to use its access to the raw diamonds - whose production is worth an estimated £125 million a month - to obtain goods from abroad, in particular weapons.
The revelation comes at a highly sensitive moment for Zimbabwe, where the future of joint government by Mr Mugabe's ruthless Zanu-PF party and the former opposition Movement for Democratic Change (MDC), led by Morgan Tsvangirai, the prime minister, hangs in the balance.
Last week Mr Tsvangirai was in Europe to press for international sanctions against Zimbabwe to be eased, despite continued repression of opponents by state security forces and the refusal by Zanu-PF to honour all the elements of its agreement with the MDC.
The MDC leader was briefed about the continued presence of the army at the diamond fields and the construction of the secret runway. A party insider said: "We know about it and it is extremely sensitive. We are very worried about what we have found out this week."
China has long been Zimbabwe's main source of arms, but delivery has been more difficult since a shipment was blocked in South Africa three years ago.
Other deliveries have come in through the Mozambican port of Beira, but government officials in the country's capital, Maputo, have expressed concern over the issue.
The army has also been frustrated in its attempts to buy weapons by Zimbabwe's finance minister, Tendai Biti, a member of the MDC who has blocked new arms purchases since taking control of the treasury under last year's power-sharing deal.
But the new facility would give Zimbabwe's Joint Operations Command, the military top brass who long swore they would never recognise Mr Tsvangirai's authority, a way to obtain weapons independently.
A Western diplomat claimed the head of Zimbabwe's armed forces, Constantine Chiwenga, had been "very busy" with the Chinese recently, adding: "We are concerned he is buying weapons."
A senior political source who has seen the pictures said: "Zanu-PF believes these diamond fields will allow it to continue to defy outstanding issues of the political agreement.
"Zanu-PF only went into the inclusive government because it lost the elections but it has no intention of fulfilling the political agreement, and wants to go it alone. But it needs an income to ensure loyalty among soldiers and other security forces."
The source said building such a runway in the mining zone did not otherwise make sense, adding: "We should all be very worried about this."
The diamond fields, in Marange district in eastern Zimbabwe, could be worth billions of pounds and make a vital contribution to rebuilding a country brought to ruin by Mr Mugabe's economic mismanagement. Tens of millions of pounds worth of the gems are smuggled into nearby Mozambique each month, to be bought by dealers from Lebanon, Belgium, Iraq, Mauritania and the Balkans - many of them with the connivance of the army and police.
The mines, whose rough diamonds have a characteristic and unappealing grey appearance, cover an area of 10 square miles. A British company, African Consolidated Resources (ACR), has a legal claim to them under a deal originally struck with the Zimbabwean government, but in 2006 the Mugabe regime went back on the agreement and declared the mines open to all comers.
Thousands of desperate Zimbabweans descended on the area to pan for diamonds among the soft red mud, and police exploitation and corruption soared.
Then, towards the end of 2008, hundreds of soldiers were sent in to evict the miners by force and take control for the army. Human rights groups estimate that between 200 and 400 were shot dead, and many more were beaten, tear-gassed, mauled by dogs or raped.
The international diamond watchdog, the Kimberley Process Certification Scheme, has been urged to suspend Zimbabwe over the violence and allegations of continuing violance at the mines - a move which would make the trade in Zimbabwean diamonds illegal. But at a meeting in Namibia in November the body decided to give Zimbabwe until June to improve, to the fury of campaigners who said the mines' output should be considered "blood diamonds" and banned.
Mark Canning, Britain's ambassador to Zimbabwe, confirmed that diplomats are closely watching developments in the diamond fields, including apparent construction of a 2,000-yard runway.
"The situation in Marange is of continual concern," he said. "What this particular facility [the runway] is, at this stage is anyone's guess, but it's crystal clear that the proceeds of a rich diamond field which has the potential to transform the fortunes of this country are being channelled into a handful of well lined pockets."
The Zanu-PF defence minister, Emmerson Mnangagwa, denied knowing of any runway under construction in the area. "Ask the mining ministry or home affairs, they might know about it," he said.
The mining minister, Obert Mpofu, who is also a member of Mr Mugabe's party, said he was on holiday and therefore could not comment.
The government says the army has withdrawn from the mining concession area and the mines are now being run by the Zimbabwe Mining Development Corporation (ZMDC), ignoring a high court order granting that right to ACR, the company which says it has the licence.
One of the mining companies involved in the development says that it is building the runway in order to comply with Kimberley Process rules that diamonds be transported in the most secure way possible, and that a private contractor is responsible for security.
ACR's chief executive Andrew Cranswick said: "We are certain the majority of valuable stones from our sites have been and are being smuggled out of Zimbabwe."
