The Petermann Glacier in northern Greenland has calved an iceberg twice the size of Manhattan, scientists say.

Images from a Nasa satellite show the island breaking off a tongue of ice that extends at the end of the glacier.

In 2010 an ice island measuring 250 square km (100 square miles) broke off the same glacier.

Glaciers do calve icebergs naturally, but the extent of the changes to the Petermann Glacier in recent years has taken many experts by surprise.

"It is not a collapse but it is certainly a significant event," Eric Rignot from Nasa said in a statement.