An iceberg twice the size of Manhattan has broken free from Greenland's massive Petermann Glacier. Scientists have said that the move could speed up the march of ice into northern waters.

This is the second time in less than two years that the Petermann Glacier has calved off another large chunk of ice. In 2010, an enormous island, roughly 97 square miles in size, broke off the glacier, along the north western coast of Greenland.

The glacier lost about one-quarter of its 40-mile long floating ice shelf, the Northern Hemisphere's largest.

The latest break was observed by NASA's Aqua satellite, which passes over the North Pole several times a day, and was noted by the Canadian Ice Service.

NASA images showed the iceberg calving - breaking off from a floating river of ice called an ice tongue, part of the land-anchored Petermann Glacier. It then moved downstream along a fjord on Greenland's north western coast.

On Monday, a crack was evident, while on Tuesday, the satellite spied a bigger gap between the glacier and the iceberg, NASA said online.

One difference between the 2010 event and this one is that the present ice island broke off further upstream, where the ice was right up against the fjord's rocky side walls, effectively damming the glacier's seaward movement.

The movement of this huge amount of ice into open water will apparently have no immediate impact on sea levels.