The whereabouts of more than a dozen whales that were trapped in a dense sea of ice off the coast of Hokkaido in northern Japan on Feb 6 are now unknown.

Town officials said they hoped the animals had freed themselves, as the ice floes appeared to have loosened on Feb 7.

The killer whales, also known as orcas, had been seen struggling and poking out of a small gap in the surface of the water in clips aired by public broadcaster NHK.

“I saw about 13 killer whales with their heads sticking out of a hole in the ice,” Mr Seiichiro Tsuchiya, who filmed the video which was posted on the Facebook page of Wildlife Pro LLC, told NHK.

The marine life expert was in the area conducting research on the local sea lion population.

“They seemed to be struggling to breathe, and it looked like they included three or four calves,” said Mr Tsuchiya.

Japan Coast Guard officials said they were notified by local fishermen on the morning of Feb 6 that the orcas were trapped between ice floes off Rausu town in eastern Hokkaido. Ice floes are sheets of floating ice.

However, officials from Rausu said they had no way to rescue the animals, which were spotted about 1km offshore.

“We have no choice but to wait for the ice to break up and for them to escape that way,” said a Rausu official.

Local weather officials also said the ice floes may have been made bigger as more waves froze.

By the morning of Feb 7, the orcas were not seen in the same spot. It was not clear what happened to them, though a pod of at least 17 orcas were sighted trapped in similar fashion 2km away six hours after the first sighting, NHK reported.

Orcas are marine mammals and can hold their breath for up to 15 minutes underwater, according to the United States-based Centre For Whale Research.

In 2005, a group of orcas was trapped in a similar way and most of them reportedly died, according to NHK, citing Rausu town officials.

Rausu is a popular whale-watching spot in Japan and drifting sea ice is a common sight during winter.