TOKYO (REUTERS) - A visit on Friday (Aug 2) by Russian Prime Minister Dmitry Medvedev to an island claimed by both Japan and Russia was extremely regrettable, Japan's foreign ministry said, urging Moscow to take constructive steps to advance ties.

In his first visit since 2015, Mr Medvedev travelled to one of four Russian-held islands off Japan's northern region of Hokkaido, known as Iturup in Russian and Etorofu in Japanese, despite protests from Tokyo ahead of the visit.

Japan claims the islands, which it calls the Northern Territories. They were invaded by the then Soviet army in the waning days of World War II.

Visits like Mr Medvedev's are incompatible with Japan's position on the Northern Territories and hurt the feelings of its people, the Ministry of Foreign Affairs said in a statement.

"We strongly urge the Russian side to take constructive measures to further advance Japan-Russia relations, including the issue of the conclusion of the peace treaty," it added.

Mr Medvedev has visited the islands before, but this week's trip could impede talks towards a peace treaty and joint economic activities there.

It comes barely a month after Russian President Vladimir Putin said the neighbours had moved towards establishing such activities.

At a conference in the Russian city of Vladivostok last September, Mr Putin caught Japanese Prime Minister Shinzo Abe off-guard when he suggested signing a peace treaty by year-end "without any pre-conditions".

Although Mr Abe rejected that proposal, he vowed in January to push for a treaty at a meeting with Mr Putin that month, but progress has proved elusive.

Mr Putin and Mr Abe met on the sidelines of Group of 20 meetings in the western city of Osaka on June 29 and discussed the treaty as well as economic activities on the islands, which Russia call the Southern Kurils.

Senior officials and political leaders from both countries have since held talks to discuss ways to end the decades-old territorial dispute and conclude the treaty.