NASA has reported an iceberg bigger than Chicago breaking into the ocean off Antarctica.

Known as B31, it is one of the biggest on the planet at 255 square miles (660 sq km) and up to 500 metres thick.

NASA has been monitoring Antarctica's Pine Island Glacier by satellite since a crack was spotted in 2011.

It is feared it could adversely contribute to rising sea levels with the potential to increase water levels by 1.5 metres.

NASA glaciologist Kelly Brunt said: "It's one that's large enough that it warrants monitoring."

While the enormous iceberg is not located in a busy shipping area at the moment. Dr Bethan Davies, a research scientist at the University of Reading, said it could head into areas with more ships.

"It's floating off into the sea and will get caught up in the current and flow around the Antarctica continent where there are ships," she told Sky News.

In a statement released by NASA, Grant Bigg, from the University of Sheffield said: "The iceberg is now well out of Pine Island Bay and will soon join the more general flow in the Southern Ocean.

"We are doing some research on local ocean currents to try to explain the motion properly. It has been surprising how there have been periods of almost no motion, interspersed with rapid flow."

Robert Marsh, a scientist at the University of Southampton, said that an iceberg of this scale could take a year or more to melt.

The largest iceberg ever recorded was called B15 which covered an expanse of 4,250 square miles (11,000 square kilometres) almost the size of Jamaica.

It broke off Antarctica's Ross Ice Shelf in March 2000 and still exists in several parts around the Antarctic.