The MQ-4C Triton drones will boost the intelligence gathering capabilities of the US-Japan defence alliance, Gen Nakatani said.

Beijing has ramped up pressure on self-ruled Taiwan in recent years and held military drills that analysts see as preludes to a possible future takeover of the island.

"The security environment surrounding our country is becoming increasingly severe," Nakatani told reporters.

He said several Tritons would arrive at the US Kadena Air Base on Okinawa's main island in the coming weeks "in order to strengthen intelligence gathering, surveillance and reconnaissance activities around Japan".

Around 54,000 US military personnel are stationed across Japan, mostly in Okinawa.

Over the past year, the Japanese military scrambled jets 30 times to intercept drones in the country's southern waters believed to have come from China, local media reported.

That is far higher than the nine times in the 2023 fiscal year and four times in fiscal 2021, according to Japan's Yomiuri Shimbun daily.

The MQ-4C is a "high-altitude, long-endurance" aircraft that can operate for over 24 hours and cover a range of 7,400 nautical miles (13,700 kilometres), its producer Northrop Grumman says.

Japan's military, which also has high-altitude drones, is separately planning to purchase smaller attack drones as a part of a broad effort to upgrade national defence.

US President Donald Trump is pressuring members of the mainly European NATO alliance to increase their military spending.

At the same time, Trump wants Asia-Pacific allies to beef up their militaries to help confront China and contain North Korea.

NATO chief Mark Rutte called Beijing's expansion of its armed forces "staggering" as he began a visit to Japan on Tuesday.

"Let us not be naive about China," Rutte told the Japan Times in an interview published Monday.

"The build-up of their armed forces and investments in their defence industry, and in their defence capabilities, is staggering."