An international humanitarian group says Myanmar's junta has used two ships that Japan donated as development assistance for military purposes.

Human Rights Watch is calling for Japan to immediately suspend non-humanitarian aid to the country and sanction junta officials over human rights violations.

The group said it obtained documents showing the military used the ships to carry over 100 soldiers and supplies to a town in the western state of Rakhine on September 14. It says the voyage and its destination were classified as top secret, suggesting the officials recognized that such use was problematic.

The Rakhine state government owns vessels that Japan delivered as part of official development assistance between 2017 and 2019. Japan's foreign ministry said on its website that the project aimed to improve nautical transportation in Myanmar.

In Rakhine State, fighting has intensified since August between the junta and ethnic-minority militants backing the country's pro-democracy forces. Many people have been forced to flee their homes.

The Japanese embassy in Yangon says it has asked Myanmar officials for an explanation but it has not had any response. It says the reported use of the ships is extremely regrettable as the assistance framework bans the use of any aid for military purposes.