This NASA satellite image shows an island of ice, 251 square kilometres in size, breaking away from the rest of the Petermann Glacier on Aug. 5. (NASA/MODIS)

For the second time in a month, a massive iceberg has broken off a glacier in Greenland, raising concerns among scientists about the world's warming oceans.

The 251-square-kilometre "ice island" broke off the Petermann Glacier, on Greenland's northwestern coast, on Aug. 5, officials with NASA said Monday.

Trudy Wohlleben, a senior forecaster with the Canadian Ice Service, detected the unusual activity while scanning imagery of the Arctic from one of NASA's satellites.

"The sheer size of it kind of took us by surprise," Wohlleben told CBC News.

The newly calved iceberg is about five times the size of Iqaluit, the capital of Nunavut.

Largest in 48 years

Researchers with the University of Delaware say it is the largest to form in the Arctic since 1962.

"I knew when I saw the picture that it was big," said Andreas Muenchow, an associate professor of physical ocean science and engineering with the Newark-based university.

The latest discovery comes after NASA imagery showed another iceberg, about seven square kilometres in size, had broken off the Jakobshavn Isbrae glacier along Greenland's west coast on July 6 and 7.

Muenchow said that while the size of the new iceberg is impressive, he is even more concerned about how the ice is melting below the Petermann Glacier's surface.

"The breakoff is not that important. … Most of the ice is being lost to the ocean underneath because the ocean's much warmer," he said.

"But it's melting from below by the ocean. It's just not as dramatic and not as visible, but that really is the main story."

Muenchow says the Petermann Glacier, one of Greenland's largest remaining glaciers, is losing 80 per cent of its mass underwater.

Ice chunks from the newly formed glacier are expected to reach Canada's coast in the next two years, although they are likely to break up into smaller pieces by then.