Japan's air force said on Wednesday jet fighter scrambles had reached a level not seen since the height of the Cold War three decades ago as Russian bombers probe its northern skies and Chinese combat aircraft intrude into its southern air space.

In the year through March 31, Japanese fighters scrambled 944 times, a 16-percent increase on the same period the previous year, the country's Self Defence Force said.

That is the second highest number of encounters ever recorded over a 12-month period since records began in 1958 and only one less than a record 944 scrambles in 1984.

"It represents a sharp increase," an SDF spokesman told a news briefing.

While not a direct measure of Russian and Chinese military activity, the numbers nonetheless point to an increase in operations by Japan's two big neighbours. The spokesman said an increased number of Chinese planes had flown through Japanese air space into the Western Pacific.

Russia's activity has gathered pace since it annexed Crimea from Ukraine last year and comes at a time when Japan faces an increasingly assertive China, which boosted its defence spending by more than 10 percent this year after a nearly unbroken two-decade run of double-digit rises.

The commander of U.S. forces in the Pacific, Admiral Samuel Locklear, said Russian activity had returned to near Cold-War levels in the past few months.

He said there was a greater Russian presence in both Northeast and Southeast Asia and that Russian Bear bombers and reconnaissance aircraft regularly flew missions in the Sea of Japan and to Alaska and the U.S. West Coast.

Locklear told a Washington congressional hearing Russian warships were deployed in the Asia-Pacific and the expected fielding of Moscow's new Borei-class submarine later this year and upgrade to land-based ballistic missiles would modernize its nuclear capability in the region.

Russian bombers and patrol planes often enter Japan's northern air space close to Hokkaido and to four smaller islands claimed by both countries.

Chinese fighter incursions are concentrated in the East China Sea, close to islets claimed by Japan and China.

Japan has increased defence spending, but by a smaller margin than China, to buy longer-range patrol aircraft, cargo jets, helicopter carriers and troop carrying Boeing V-22 Ospreys and Lockheed Martin F-35 stealth fighters.

Prime Minister Shinzo Abe also plans to loosen constitutional constraints on Japan's defence forces, something that will allow them to operate more freely overseas and deepen cooperation with U.S. forces. (Reporting by Tim Kelly; Additional reporting by David Brunnstrom in Washington; Editing by Nick Macfie and Chris Reese)