The Arctic ice cap keeps melting under the effects of global warming, and has shrunk by the second-largest amount in any northern summer since satellite observations began 30 years ago, according to US scientists.

Measurements on Tuesday showed the size of the ice cap at 5.26 million sqkm, just below the 5.32 million sqkm observed on September 21, 2005, the National Snow and Ice Data Centre said.

Since the start of the month, the centre, based in Boulder, Colorado, said the Arctic polar cap shrank by 2.06 million sqkm.

The melting is so fast and extensive it could reduce the ice cap below the 4.25 million sqkm attained in the northern summer of 2007, the smallest coverage yet observed by satellites.

Since the end of the Arctic summer and the start of freezing autumn were several weeks away, the ice cap could dwindle even more than it did last year.

At the end of last summer in the northern hemisphere, the Arctic ice cap was 40 per cent smaller than the average of 7.23 million sqkm observed between 1979 and 2000.

The North Pole melting season begins in mid-June.

The ice cap shrinks to its smallest area by mid-September and spread to its maximum extent by mid-March.

The centre said, ''The bottom line, however, is that the strong negative trend in summertime ice extent characterising the past decade continues.''

The North Pole could even become ice-free by next month for the first time in modern history, setting a new milestone in the effects of global warming on the Arctic ice shelf, a glaciologist at the centre, Mark Serreze, said.

''We could have no ice at the North Pole at the end of this summer. And the reason here is that the North Pole area right now is covered with very thin ice, and this ice we call 'first-year ice', the ice that tends to melt out in the summer.''

Mr Serreze estimated it as a 50:50 proposition that the ice cap could vanish. If it did happen in September, ''it's possible that ships could sail from Alaska right to the North Pole''.

The Arctic had been free of ice in the Earth's geological history, but never in modern times, Mr Serreze said.

''Clearly, if you look over what we have seen in the past three years and where we were headed, we are in ... this long-term decline and we may have no ice at all in the Arctic Ocean in summer by 2030 or so.'' AFP