Russian scientists on Wednesday announced they had drilled deep through Antartica's icesheet to reach a pristine lake untouched for tens of thousands of years.

"A small window has opened into the unknown world of Lake Vostok," Russia's Arctic and Antarctic Institute said in the first official confirmation of the breakthrough.

"For me, discovering this lake is comparable to the first flight to space. In its technical complexity, its importance and its uniqueness," expedition leader Valery Lukin told the Interfax news agency.

Specialists "penetrated the prehistoric waters of Lake Vostok under the ice through a deep ice borehole," scientists said, revealing a discovery that took place on February 5.

The expedition drilled down to the lake's surface at a depth of almost four kilometres (2.34 miles) but did not immediately take a water sample to avoid contamination, Lukin said.

"According to the ecologically clean technique we have developed, now we cannot take any samples," Lukin said, explaining the water would get contaminated with kerosene and freon being used antifreeze.

Instead scientists will wait for a column of water to rise up through the borehole and freeze, he said. Scientists plan to collect the water samples from December this year to January 2013.

He said scientists could not take the samples back to Russia due to restrictions on liquids on flights, so they would be taken back on a research ship, arriving only in May 2013.

Lake Vostok, the largest subglacial body of water in Antarctica, "presents a potentially a unique water ecosystem," isolated from the atmosphere for at least one million years, the Arctic and Antarctic Institute said on its website.

Nevertheless the expedition has raised fears among international experts about the environmental impact of the kerosene used as antifreeze and questions about the scientific worth of the mission.

Russia has presented the expedition as a major triumph, and the Minister of Natural Resources and Ecology Yury Trutnev viewed the site earlier this month.