The privately owned Save Valley Conservancy group says only people who are part of President Robert Mugabe's inner circle stand to benefit from the land grab, while the reserve and the animals, including endangered species, would suffer.
Lions, leopards, elephants, cheetahs and the often-poached rhino all live on the land. As part of the seizure, hunting licences are being granted to politicians, in a move that has conservationists worried.
The plan is regarded as one of the largest seizures since 2000, when the Zimbabwe government began to kick white farmers off their land and transfer ownership to blacks.
The land reform programme was meant to rectify colonial-era imbalances which heavily favoured the white minority. However, critics say it has largely benefited politically connected elites from Mugabe's Zanu-PF party.
"We are all very concerned," one diplomat told Deutsche Presse-Agentur, speaking on condition of anonymity. "We are considering appropriate reactions. It's a very serious situation."
One measure could be the withdrawal of support for a United Nations' World Tourism Organisation congress next year, being hosted jointly by Zimbabwe and Zambia at Victoria Falls, a tourist site on the shared border.
"Zimbabwe depends entirely on international support for the congress," said a European diplomat. "It cannot go ahead without us."
The idea would be discussed in Berlin this week, during a meeting of German officials, the diplomats said.
Zimbabwe's state wildlife authority announced August 9 it was granting 25 top officials from Zanu-PF control over most of the Save (pronounced Sa-Veh) reserve, which covers 2 600 square kilometres in the country's arid south-east.
Running along the banks of the Save river, the conservancy – respected as a leader in wildlife management and research – is collectively controlled by international investors, white ranchers who formerly ran cattle on the land, local black businessmen and hundreds of rural farmers.
"It is a working example of how something really special can be a success, by including all sectors of the community, especially the rural poor who have previously got nothing out of wildlife," said Wilfried Pabst, a German businessman who is vice-chair of the conservancy.
Pabst rejected accusations by the government that the reserve is opposed to ensuring a fair deal for blacks.
"Two-thirds of stakeholders of the conservancy are black. It is now being threatened by a collection of greedy individuals who are bringing nothing into the conservancy and will destroy it," Pabst said.
The Save Valley Conservancy noted in a statement that the government had supported the reserve consistently since the reserve was founded in 1991.
Conservationists warn that two other Zimbabwean reserves that were the subjects of takeovers have since collapsed. Some 600 workers at Save stand to lose their jobs there should it suffer a similar fate, they say.
The Save group also warned that, already, anti-poaching staff were being removed from their posts and the illegal hunting of antelopes was on the rise for sale on local meat markets.
Diplomats said they were in talks with Zimbabwe's government in an effort to reach a conservation deal. – Sapa-dpa