Japan scrambled Air Self-Defense Force jets over one thousand times in the 2016 fiscal year, the most on record, partly due to growing tensions with China over ownership of the Senkaku Islands.

The Senkaku Islands, which China calls the Diaoyu, are an uninhabited set of islands in the resource-rich East China Sea.

According to the Japanese Defense Ministry, the Air Self-Defense Force scrambled jets 1,168 times in the 2016 fiscal year, which ends in March. The Japan Times notes that, in nearly three out of four of those occasions, the jets were responding to Chinese aircraft flying too close to Japanese airspace. Nearly all other incidents involved Russian aircraft. The outlet adds that the previous record was in 1984 before the fall of the Soviet Union.

The Defense Ministry noted an additional 26 monitored “unusual” flights that did not violate Japan’s airspace but raised concerns and merited documenting. They noted that no aircraft illegally entered Japanese airspace this year.

At a press conference announcing the new numbers, Admiral Katsutoshi Kawano, whom Reuters describes as “Japan’s top military commander,” blamed increased military activity on the part of neighboring China. “Recently we have seen Chinese military aircraft operating further south and that is bringing them closer to the main Okinawa island and other parts of the island chain,” he explained.

Many incidents not near Okinawa – where the U.S. military also operates – occurred near the Senkaku Islands.

China established an Air Defense Identification Zone (ADIZ) over the Senkakus in 2013, which required all non-Chinese aircraft to identify itself to the Chinese government if entering the airspace over the islands. Japan rejected the ADIZ, interpreting the need for Japanese aircraft over Japanese territory to identify itself to China as an insult and ignored it.

The United States has also ignored it and repeatedly warned China that any attempts to enforce the ADIZ against Japan using military force would trigger a military response from the United States, which is treaty-bound to retaliate against any attacks on Japan.

U.S. Defense Secretary James Mattis most recently reiterated this warning during his visit to Tokyo in February. “I made clear that our long-standing policy on the Senkaku Islands stands – the US will continue to recognize Japanese administration of the islands and as such Article 5 of the US-Japan Security Treaty applies,” Mattis said, speaking alongside Japanese Defense Minister Tomomi Inada.

The Chinese government responded to those claims by reiterating their assertion that the Diaoyu belong to China. “No matter what anyone says or does, it cannot change the fact that the Diaoyu Islands belong to China, and cannot shake China’s resolve and determination to protect national sovereignty and territory,” Chinese Foreign Ministry spokesman Geng Shuang said at the time.

Following Mattis’s visit to Tokyo – and Prime Minister Shinzo Abe’s visit to America to meet with President Donald Trump – Japan announced an initiative to encourage Japanese nationals to inhabit the Senkaku Islands, along with another 143 such islands that have been suffering from population decline or never had a large population to begin with.

The Senkaku Islands are in the latter category, and having any population at all would allow Abe to designate the territory as Japanese and put it on a census map, making China’s claim harder to push on the international stage.

Fox News reported at the time of the initiative that the Japanese government was seeking to begin “the construction of civic facilities, the purchasing of land, the improvement of ports and stopping foreign vessels from illegally visiting the islands.”

Japan has threatened to bring China to an international tribunal to end the dispute over the Senkakus. China recently lost a major international case at the Permanent Court of Arbitration at the Hague over islands in the South China Sea.

In the Philippines v. China case, the court ruled in July 2016 that the Spratly Islands, Paracel Islands, and a number of other territories in the South China Sea that Beijing claimed were not legally Chinese territory, but instead belonged to a variety of countries whose land and sea China had usurped, among them Vietnam, the Philippines, Malaysia, and Brunei.