Japanese Prime Minister Shinzo Abe has criticised his Russian counterpart Dmitry Medvedev's weekend visit to disputed islands, intensifying a long-running dispute over the archipelago.
"[The visit] conflicts with Japan's position and hurts the feelings of the Japanese people. It is extremely regrettable," Abe told a parliamentary panel when asked about the trip.
Abe's comments come after the Russian prime minister on Saturday arrived in Iturup, one of four in the Kuril island chain that lies off Russia's far eastern coast and just north of Japan.
Japanese Foreign Minister Fumio Kishida immediately summoned Russia's ambassador to Japan in protest at the visit to the islands, which Japan calls the Northern Territories.
The visit has also led to Kishida postponing a planned trip to Russia in the next few weeks.
Kishida had been expected to prepare the ground for Russian President Vladimir Putin to make a long-delayed visit to Japan this year to seek a breakthrough in the 70-year-old territorial dispute.
Soviet troops seized the islands just after Japan surrendered in World War II.
The seven-decade-old dispute has hampered trade and prevented Moscow and Tokyo from signing a formal post-war peace treaty.
"What is significant for our country's national interests is to solve the territorial issue of the Northern Territories and sign the peace treaty," Abe said.