A massive crack is growing wider in the Antarctic ice sheet and could break apart in the coming months, forming an iceberg the size of New York City, NASA scientists warned on Thursday.

The crack in western Antarctica's Pine Island Glacier stretches for at least 18 miles (30 kilometers) and runs 165 feet (50 meters) deep.

The rift is widening at a rate of 6.5 feet (two meters) per day, said NASA project scientist Michael Studinger.

When the ice breaks apart, it will produce an iceberg more than 340 square miles (880 square kilometers), said Studinger, who is part of the US space agency's IceBridge project.

But the process is not a result of global warming, he said.

"We expect that later this year or early next year there will be a pretty large iceberg forming as part of a natural cycle," he added.

"These are cyclical events that occur every few years. The last big calving event occurred in 2001, so in general people have been expecting something to happen like that very soon.

"For us it is very exciting to actually see this while it is happening."

The rift was first glimpsed in late September by scientists monitoring changes in the ice shelf via airplane flyovers in order to fill in the gaps left between a pair of satellites, ICESat (Ice, Cloud and land Elevation Satellite), which ended in 2009, and ICESat 2, which launches in 2016.

"A lot of times when you're in science, you don't get a chance to catch the big stories as they happen because you're not there at the right place at the right time," said John Sonntag, instrument team lead for Operation IceBridge, based at Goddard Space Flight Center.

"But this time, we were."