The North-West Passage – the sea route running along the Arctic coastline of North America, normally perilously clogged with thick ice – is nearly ice-free for the first time since records began.

"Since August 21 the North-West Passage is open to navigation. This is the first time that it happens," Nalan Koc, head of the Norwegian Polar Institute's climate change programme, told reporters in Longyearbyen, a town in the Arctic archipelago of Svalbard.

"The Arctic ice sheet currently extends on 4.9m square kilometres. In September 2005 it measured 5.3m square kilometers."

Koc was quoting research from the US National Snow and Ice Data Centre, where scientists monitor the surface of the Arctic ice sheet at regular intervals. Last week they noted "the imminent opening" of the North-West Passage.

"Analysts confirm that the passage is almost completely clear and that the region is more open than it has ever been since the advent of routine monitoring in 1972," they said in research conclusions published on the centre's website.

The news is yet another milestone showing that the Earth is warming up. The route was first navigated in the early 1900s by the Norwegian explorer Roald Amundsen, who later beat Robert F Scott in the race to the south pole in 1911. Amundsen and his crew took nearly two years to pick their way through a labyrinth of narrow lanes of open water and thick ice. Now it would be comparatively plain sailing.

It has long been expected that Arctic sea routes including the North-West Passage and its north-east counterpart along the coast of Siberia will become more passable as the Earth's temperature climbs.

Both are considered strategic cargo routes because they are shortcuts between the northern parts of the Atlantic and Pacific oceans.

In recent years Russia, Canada and Norway have turned their attention to their northernmost regions. A quarter of all untapped reserves of oil and gas are to be found in the Arctic, according to the US Geological Survey. Already the Russian gas giant Gazprom is eyeing the exploitation of the world's largest gas offshore field, Shtokman, off the coast of northern Siberia.

This month Russia planted a flag on the sea bed under the north pole to symbolically claim the region. Canada and Denmark, meanwhile, are embroiled in a battle over sovereignty over Hans Island, a speck of land off Greenland.