After two Russian fighter jets entered Japanese airspace on Thursday, the Japanese military scrambled their fighter jets in a quick response.

According to Reuters, Japan claims that the two Russian jets entered Japanese airspace near the main northern island of Hokkaido and the disputed Kuril Islands early Thursday morning. The Kuril Islands have been part of a territorial dispute between Russia and Japan since the end of World War II.

The Japanese military almost immediately scrambled fighter jets in an attempt to respond to the possible aggression, Tokyo said. Russian fighter planes quickly left Japanese airspace and no further incident occurred.

According to CNN, while the Japanese government insists that the Russian fighter jets had indeed entered Japanese airspace, Russia denies it.

The Kuril Islands remain a difficult subject in relations between Japan and Russia. The Soviet Union seized the island during World War II, and the San Francisco Peace Treaty of 1951 called for Japan to give up its claim on the islands, though it did not recognize the Soviet Union’s sovereignty either. Japan still claims them, and Thursday was “Northern Territories Day,” when the issue is commemorated. Prime Minister Shinzo Abe spoke at a rally calling for both improved relations with Russia and the return of the Kuril Islands.

"I had telephone talks with President (Vladimir) Putin in December, and told him I would like to work to find a mutually acceptable solution to this last-remaining major problem between Japan and Russia," Abe said, according to Reuters.

"The government intends to follow its basic policy of settling the territorial issue and then sign a peace treaty. We will press ahead with negotiations with strong will so that progress will be made towards the conclusive resolution of the territorial problem."