Two Chinese fighter jets carried out an "unsafe" intercept of a US military reconnaissance aircraft over the South China Sea, the Pentagon said, a further escalation of tensions in and around the contested waterway.

The incident took place in international airspace as the US maritime patrol aircraft carried out "a routine US patrol," a Pentagon statement said.

The incident comes a week after China scrambled fighter jets as a US Navy ship sailed close to a disputed reef in the South China Sea.

Another Chinese intercept took place in 2014 when a Chinese fighter pilot few acrobatic manoeuvres around a US spy plane.

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The intercept is also days before US President Barack Obama travels to parts of Asia from May 21-28, which will include a Group of Seven summit in Japan and his first trip to Vietnam.


ABOUT THE SOUTH CHINA SEA

1 China claims most of the South China Sea, through which US$5 trillion in ship-borne trade passes every year.
2 The Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims.
3 Washington has accused Beijing of militarising the South China Sea after creating artificial islands
4 Beijing, in turn, has criticised increased US naval patrols and exercises in Asia.


The Pentagon statement said the Department of Defence was addressing the issue through military and diplomatic channels.

"Over the past year, DoD has seen improvements in PRC actions, flying in a safe and professional manner," the Pentagon statement said, using an acronym for the People's Republic of China.

The Chinese Embassy in Washington did not immediately respond to a request for comment.

In 2015, the United States and China announced agreements on a military hotline and rules of behaviour to govern air-to-air encounters called the Code for Unplanned Encounters at Sea (CUES).

"This is exactly the type of irresponsible and dangerous intercepts that the air-to-air annex to CUES is supposed to prevent," said Greg Poling, director of the Asia Maritime Transparency Initiative at Washington's Centre for Strategic and International Studies think-tank.

Poling said either some part of China's Air Force "hadn't gotten the message", or it was meant as a signal of displeasure with recent US freedom of navigation actions in the South China Sea.

"If the latter, it would be very disappointing to find China sacrificing the CUES annex for political gamesmanship."

The Pentagon last month called on China to reaffirm it has no plans to deploy military aircraft in the Spratly Islands after China used a military plane to evacuate sick workers from Fiery Cross Reef, where it has built a 3000m runway.

In April 2001, an intercept of a US spy plane by a Chinese fighter jet resulted in a collision that killed the Chinese pilot and forced the American plane to make an emergency landing at a base on China's Hainan Island.

The 24 US air crew members were held for 11 days until Washington apologised for the incident. That encounter soured US-Chinese relations in the early days of President George W Bush's first Administration.

- Reuters, AAP