A crack that could create an iceberg the size of Cyprus - and destabilise one of the largest ice shelves in the Antarctic - has branched out and begun to widen more quickly.

Dan McGrath, a geophysicist with the US Geological Survey, says the new fissure has turned toward the shelf's ocean edge, potentially speeding up the iceberg's process of breaking off.

The new crack, on the Larsen C ice shelf, is an offshoot of a rupture that gained notice after growing dramatically in 2014.

Last year it was forecast to cause the separation of a 5,000-square-kilometre iceberg within years.

The original crack had been widening at a rate of about a metre per day - that rate has increased since the new fissure formed.

Dr McGrath, who has studied the shelf extensively, said the combination of the new crack and the faster widening could point to an imminent separation of the berg, even as soon as this summer.

Once this occurs, a roughly 180km-long iceberg will be released - one of the largest ever recorded.

The loss of so much ice would shrink the shelf by about 10% and scientists have warned that the loss of such a large mass of ice would create a "significant risk" of the shelf as a whole becoming unstable and breaking up.

There is now only about 20km of ice tethering the Larsen C ice shelf to Antarctica.