Japan's coast guard says Russia has issued restrictions on the passage of vessels other than Russian ships in waters near four Russian-held islands from April 11 through 17.

Japan claims the four islands, which it calls the Northern Territories. 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.

The Japanese government has lodged a protest with Russia, saying the restrictions are unacceptable since the islands are Japan's inherent territory.

The United Nations Convention on the Law of the Sea allows ships to pass through the territorial waters of another state as long as they do not threaten the security of the coastal state.

The convention also allows a coastal state to take the steps to block passage of foreign ships in its territorial waters if the country considers doing so necessary.

But Japan's foreign ministry says the restrictions go against Japan's stance on the Northern Territories.

Japan will further analyze the Russian move. Some government officials say Russia restricted the passage of foreign ships as it wants to show that the islands are its territory. Others say it is a response to the leaders of Japan and the US agreeing to strengthen defense cooperation during their recent summit.