Satellite images show the ice cap had melted to 3.4 million square kilometres as of September 16, the predicted lowest point for the year.

That's the smallest Arctic ice cover since record-keeping began in 1979, the National Snow and Ice Data Centre said. "We are now in uncharted territory," the centre's director, Mark Serreze, said.

"While we've long known that as the planet warms up, changes would be seen first and be most pronounced in the Arctic, few of us were prepared for how rapidly the changes would actually occur."

Arctic sea ice expands and contracts seasonally, with the lowest extent usually occurring in September. This year's minimum followed a season already full of records for shrinking ice, with lowest ever extents recorded on August 26 and again on September 4.

In the last two weeks, the ice cover melted by more than 518000km², quite a large margin for the end of the summer, the NSIDC said.

"The strong late season decline is indicative of how thin the ice cover is," said NSIDC scientist Walt Meier. "Ice has to be quite thin to continue melting away as the sun goes down and autumn approaches." Scientists use Arctic sea ice extent as an indicator of what's happening with the overall climate. Despite year-to-year fluctuations from natural weather variations, the ice cap had shown a clear trend towards shrinking over the last 30 years, the NSIDC said.

"This year's minimum will be nearly 50% lower than the 1979 to 2000 average," it noted. The Colorado-based centre said the Arctic was shifting in composition. Whereas previously most of the ice stayed frozen through several summers, now much of it melts and refreezes each season.

"Twenty years from now in August you might be able to take a ship right across the Arctic Ocean, once blocked year-round by ice," said NSIDC scientist Julienne Stroeve.

Climate models predict "ice-free conditions" before 2050, she added, but said the decline appeared to be happening faster than predicted.

The NSIDC warned increased heat and moisture from the melting Arctic ice cover could have global climate implications.

"This will gradually affect climate in the areas where we live. We have a less polar pole - and so there will be more variations and extremes," it said.

Environmental activists Greenpeace lamented the announcement.

"In just over 30 years we have altered the way our planet looks from space, and soon the North Pole may be completely ice-free in summer," Greenpeace executive director Kumi Naidoo said.

"I hope future generations will mark this day as a turning point, when a new spirit of global cooperation emerged to tackle the huge challenges we face," he added.