Armed Danish commandoes are thought to have been landed on a giant oil rig by helicopter to prevent environmentalists interfering with a British oil company's controversial exploration of deep Arctic waters. In a stand-off in the Davis Strait, west of Greenland, the Danish navy has been shadowing the Greenpeace ship Esperanza as it tracked the 53,000 tonne Leiv Eiriksson in iceberg-strewn sea to the site where it plans to search for oil at depths of up to 5,000ft.

Greenpeace activists protect against Arctic oil exploration by the Leiv Eiriksson rig. (Photograph: Markel Redondo/Greenpeace) The confrontation between Denmark and Greenpeace, which argues that it is dangerous to drill for oil in pristine Arctic waters, follows the decision by Scottish oil company Cairn Energy to explore for oil and gas in Baffin Sea this summer.

Fears that an Arctic spill would be difficult if not impossible to clean up were confirmed in an email exchange between the British Foreign Office and the energy secretary, Chris Huhne, that was obtained by Greenpeace under freedom of information legislation. Officials briefed Huhne, saying: "It is difficult to get assistance in case of pollution problems in such areas, and near impossible to make good damage caused."

They warned of "significant" environmental challenges and the potential for a Gulf of Mexico-type spill. "The impact of such a spill in the Arctic would be proportionately higher due to the lower temperatures and (in winter) lack of sunlight that will inhibit oil eating bacteria (which played a large role in cleaning up the Macondo spill). The Arctic ecosystem is particularly vulnerable, and emergency responses would be slower and harder than the Gulf of Mexico due to the areas remoteness and the difficulty of operating in sub-zero temperatures. A situation compounded by response lag resulting from the vast distances between points of habitations and at certain times, winter ice."

Ruth Davis, chief policy adviser at Greenpeace UK said: "These documents make it clear that companies like Cairn are playing Russian roulette with one of the most important environments in the world. When even the UK government recognises the huge risks associated with the oil drilling in the Arctic then it must be time to halt the rush for oil in one of the most delicate ecosystems in the in the world."

Cairn says it has prepared comprehensive oil spill plans, and has put up a bond of $2bn. Last month it said in a statement: "Wherever it is active, Cairn seeks to operate in a safe and prudent manner. The Greenlandic Bureau of Minerals and Petroleum has established some of the most stringent operating regulations anywhere globally, which mirror those applied in the Norwegian North Sea. Cairn respects the rights of individuals and organisations to express their views in a safe manner."

Seven major oil companies have licenses to explore off Greenland but Cairn will be the only one to begin operations in the short July-October "summer window" when the ice has retreated. Cairn holds 11 licences covering over 80,000 square kilometres and plans to drill four exploratory wells to depths of around 5,000ft, the deepest ever attempted in the Arctic.

Fears that Greenpeace plan to prevent work have been heightened since the group occupied one of Cairn's drilling ships working in shallower Arctic waters last years, and 11 climbers also boarded the Leiv Eiriksson, when it left Turkey for Greenland last month. Greenpeace also tried to stop the rig as it passed Greece and Italy last month but was prevented by storms.

"We are in the Davis Straits doing 10 knots in big seas. There are icebergs everywhere. We're getting very close to where Cairn intends to drill.," said the Greenpeace campaigner Ben Ayliffe aboard the Esperanza.

Denmark is believed to have sent two warships to protect Cairn from Greenpeace, which in turn has sent two ships to monitor Cairn. Greenland is an autonomous country within the Kingdom of Denmark.