A group of former residents of Japan's Northern Territories and their families has offered prayers for their ancestors in a ceremony held at sea.

Russia controls the Northern Territories, which comprise four islands. Japan claims them. The Japanese government maintains the islands are an inherent part of Japan's territory. It says the islands were illegally occupied after World War Two.

Forty-two former residents and their families set off from Nemuro port on the northern island of Hokkaido, on Saturday morning, aboard a vessel built specifically for former islanders' visits to the territories.

Before their departure, Hokkaido Governor Suzuki Naomichi said he hoped the former residents' thoughts for their ancestors reached the islands where they are resting.

The vessel was forced to turn back about 6 kilometers offshore due to bad weather. The memorial service was held on the way back with the participants observing a moment of silence and offering prayers. The ship returned to port after one hour.

The director of the association of former residents, Waki Kimio, said even though the islands were not visible, he believes the participants were able to comfort the souls of their ancestors.

Until three years ago, the former residents of the Northern Territories and their families made annual trips to the islands to visit their ancestors' graves, as part of a bilateral exchange program.

However, the program was called off, first because of the coronavirus pandemic, and then because of Russia's invasion of Ukraine.

The association of former residents and the Hokkaido government have this year organized a series of boat tours for memorial services at sea, as an alternative.

Nine more tours are scheduled to take place through August 10. A total of 331 people from around Japan are expected to take part.