"All of these areas have previously had two metres of ice," said Schauer, who works at the Alfred Wegener Institute in Germany, of a trip from Norway around the North Pole and back towards Russia. The last major survey was in 2001.

A summer trend of increased ice melting -- widely linked to human emissions of greenhouse gases -- may also threaten the livelihoods of Arctic peoples and wildlife such as polar bears.

But it could open a fabled short-cut for ships between the Atlantic and Pacific Oceans and allow exploration for oil and gas. Russia planted a flag on the ocean floor beneath the North Pole last month in a symbolic claim.

The European Space Agency said last week the shrinking of the ice had opened the Northwest Passage north of Canada as a short-cut route between Europe and Asia "that has been historically impassable".

The passage will ice over in winter.

Schauer also said that there was only some ice blocking an alternative northern sea route along the coast of Russia. Both polar routes are far shorter between Europe and many Asian ports than via the Suez or Panama canals.

"The Northeast Passage seems ice-free north of Siberia except a little part between the mainland and (the island of) Severnaya Zemlya," she said.

The thinning adds to evidence from satellites of a shrinking of the Arctic summer ice extent to record lows. Some experts say summer ice might vanish within decades, earlier than around the end of the century projected by the UN climate panel.

The US National Snow and Ice Data Center said on Monday that Arctic sea ice had shrunk to a record low 4.14 million sq km (1.6 million sq mile), more than 1.2 million sq km -- or the size of South Africa -- less than the previous low in 2005.