An enormous iceberg, measuring more than 3000 square kilometres, is poised to detach from one of the largest floating ice shelves in Antarctica and float off in the Weddell Sea, south of the tip of South America.

Scientists have been expecting the break from the Larsen C ice shelf, monitoring the progress of a crack which extended to more than 160 kilometres long in recent months. The latest update from scientists with NASA and the University of California, Irvine found that only five remaining kilometres of ice continue to connect the impending iceberg to the larger shelf.

Those parts of the iceberg that have already detached have begun to move rapidly seaward, widening the rift in recent days and leaving the remaining ice "strained near to breaking point", according to Adrian Luckman, a scientist monitoring Larsen C at Swansea University in Wales.

The expected calving event - on its own - will not affect global sea level, because the ice that has detached was already afloat in the ocean. But some scientists fear that it could hasten the destabilisation of the larger Larsen C ice shelf.