Russia threatened Thursday to ban visa-free travel for Japanese officials to four disputed Pacific islands if they make statements demanding the territories be returned to Japan.
The renewed tension could set back a fresh drive by the countries' leaders to end a decades-old territorial dispute over the small islands north of Hokkaido, which were seized by Soviet troops at the end of World War II.
The spat over the islands, known as the Southern Kurils in Russia and as the Northern Territories in Japan, has prevented Moscow and Tokyo from signing a treaty formally ending hostilities and still hinders efforts to improve relations.
"If for some reason Japanese politicians cannot refrain from making public statements on the subject of the islands after a visit to Russian territory, we reserve the right to limit their participation in such trips," the Russian Foreign Ministry said.
It underlined in a written statement that visa-free travel was permitted for Japanese officials as a "humanitarian act," mainly to enable them to visit the graves of their ancestors.
The ministry made clear it was referring to comments by Ichita Yamamoto, Japan's Minister of State for Okinawa and Northern Territories Affairs, during a visit to the islands and quoted him as saying his view of "the need to return territories" was reinforced by his trip.
Tokyo did not immediately respond. Asked about the Russian Foreign Ministry's warning at a news conference, Japanese Chief Cabinet Secretary Yoshihide Suga said he was not aware of it and declined further comment.
A report by the Nikkei business daily on a news conference given by the minister Sept. 23 following his visit appeared to contain no inflammatory comments.
"We intend to calmly accept the current condition surrounding the Northern Territories and solve the territorial issue through tenacious negotiations, without fluctuating between hope and despair," he said, Nikkei reported.
President Vladimir Putin and Japan's then-Prime Minister Shinzo Abe agreed to revive talks on the islands during a summit in Moscow in April. Any new tensions over the islands would be likely to set back those efforts.
An end to the dispute is not in sight, but reviving long-stalled talks is a first step to improving economic cooperation, which both sides say has failed to live up to its potential.
The islands, located near rich fishing grounds, were seized by the Soviet Union, of which Russia was then the biggest part, after it declared war on Japan in August 1945 and days before Japan surrendered, forcing about 17,000 Japanese to flee.
Japan and Russia are still nominally at war, although hostilities ended shortly after Japan surrendered. The conclusion of a peace treaty depends on the resolution of the territorial dispute.