Japan’s air force scrambled fighter jets to chase away foreign aircraft at record pace in the year to March 31, government figures showed on Thursday, as Chinese military activity in and around the East China Sea escalated.

Japan worries that China’s probing of its air defenses is part of a push to extend its military influence in the East China Sea and western Pacific, where Japan controls an island chain stretching 1,400 km (870 miles) south towards Taiwan.

“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,” Japan’s top military commander, Admiral Katsutoshi Kawano, told a briefing in Tokyo.

Okinawa is home to the biggest concentration of U.S. Marine Corp forces outside the United States, hosting the bulk of the roughly 50,000 U.S. military personnel stationed in Japan.

Japan’s Air Self Defence Force reported its fighters scrambled 1,168 times over the 12 months, up from 873 last year. A record 851 jets headed off approaching Chinese planes, or 280 more instances than in the corresponding period last year.

The new figure was also well above the previous high of 944 incidents in 1984, when Russian, rather than Chinese, aircraft triggered most of the scrambles.

The uptick in Chinese activity has contributed to rising tension in East Asia since the start of the year as North Korea pushes ahead with ballistic missile and nuclear bomb tests that have stoked fears in Japan, the United States and elsewhere.

Japan’s navy plans joint drills around the East China Sea with the U.S. Navy’s Carl Vinson carrier strike group, as it steams towards the Korean peninsula, two sources told Reuters.

Encounters with Russian aircraft, which are often bombers flying from the north that skirt around Japan’s airspace, rose 4.5 percent, to 301 scrambles.

(Reporting by Nobuhiro Kubo and Tim Kelly; Editing by Clarence Fernandez)