According to media reports Thrusday, two Chinese fighter jets carried out an “unsafe” intercept of a US military reconnaissance aircraft over the South China Sea, the Pentagon has said, drawing a rebuke from Beijing, which demanded that Washington end surveillance near China.

The Chinese aircraft intercept took place in international airspace on Tuesday as the US maritime patrol aircraft carried out “a routine US patrol”, a Pentagon statement said on Wednesday.

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

The intercept occurred days before President Barack Obama travels to parts of Asia from May 21-28, including a Group of Seven summit in Japan and his first trip to Vietnam.

China claims most of the South China Sea, through which $US5 trillion ($A6.91 trillion) in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims.

Washington has accused Beijing of militarising the South China Sea after creating artificial islands, while Beijing in turn has criticised increased US naval patrols and exercises in Asia.
The Pentagon statement said the Department of Defense was addressing the issue through military and diplomatic channels.

China’s Foreign Ministry spokesman Hong Lei said the US statement was “not true” and that the aircraft had been engaging in reconnaissance close to China’s island province of Hainan.

“It must be pointed out that US military planes frequently carry out reconnaissance in Chinese coastal waters, seriously endangering Chinese maritime security,” Hong told reporters at a regular press briefing on Thursday.

“We demand that the United States immediately cease this type of close reconnaissance activity to avoid having this sort of incident happening again,” Hong said, adding that the actions of the Chinese aircraft were “completely in keeping with safety and professional standards”.

“They maintained safe behaviour and did not engage in any dangerous action,” Hong said.

China’s Defence Ministry said in a fax that it was looking into reports on the incident.

The Pentagon has yet to release the precise location of the encounter.

In 2015, the US 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 annexe 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 airforce “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.”

While the precise location of the encounter is not yet known, regional military attaches and experts say the southern Chinese coast is a military area of increasing sensitivity for Beijing.
Its submarine bases on Hainan are home to an expanding fleet of nuclear-armed submarines and a big target for on-going Western surveillance operations.

The Guangdong coast is also believed to be home to some of China’s most advanced missiles, including the DF-21D anti-ship weapon.

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 Hainan.
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.