NASA has discovered a perfectly rectangular, kilometre-long iceberg floating among a sea of ice in Antartica’s Weddell Sea.

The US space agency posted a photo to Twitter displaying the mammoth ice block, formed as it broke off an Antarctic ‘Larsen C’ ice shelf.

The rare find which has not been worn down by ocean waves contained four pointed 90 degree angles, straight edges and a flat surface.

From yesterday's #IceBridge flight: A tabular iceberg can be seen on the right, floating among sea ice just off of the Larsen C ice shelf. The iceberg's sharp angles and flat surface indicate that it probably recently calved from the ice shelf. pic.twitter.com/XhgTrf642Z

— NASA Ice (@NASA_ICE) October 17, 2018

These objects are not unknown and are often referred to as tabular bergs.

They can reach deep beneath the surface and extend hundreds of kilometres wide.

The iceberg was photographed by scientists on a research plane as part of NASA’s Operation IceBridge and is believed to be about 1.6 kilometres long.

Academic research journal Science Alert said the spotted iceberg was “perfectly natural” and unlike any normal iceberg that might be full of cracks.

Nasa glaciologist Kelly Brunt told Live Science journal that such icebergs were often geometrically shaped because they formed much like a fingernail growing too long and cracking off at the end.

“They split from the edges of ice shelves – large blocks of ice, connected to land but floating in the water surrounding iced-over places like Antarctica.

“What makes this one a bit unusual is that it looks almost like a square,” she said, adding that only 10 per cent of this iceberg was visible above the waterline.

NASA scientists took several other aerial photographs of icebergs as part of their flight over Antarctica on October 22.

From Friday's #IceBridge flight: An iceberg surrounded by water and sea ice floes in the Weddell Sea. The submerged portion of the iceberg is partially visible (in bright blue). pic.twitter.com/ZQMjUGf6h2

— NASA Ice (@NASA_ICE) October 22, 2018

In December 2017, NASA scientists came across a similarly shaped iceberg that was more than 42,000 kilometres square.

Much of the iceberg labelled A-68 was contained under water after it broke off the same Larsen C ice shelf.