WASHINGTON/BEIJING (Reuters) - Beijing demanded an end to U.S. surveillance near China on Thursday after two of its fighter jets carried out what the Pentagon said was an “unsafe” intercept of a U.S. military reconnaissance aircraft over the South China Sea.

The incident, likely to increase tension in and around the contested waterway, took place in international airspace on Tuesday as the plane carried out “a routine U.S. patrol,” a Pentagon statement said.

A U.S. Defence official said two Chinese J-11 fighter jets flew within 50 feet (15 meters) of the U.S. EP-3 aircraft. The official said the incident took place east of Hainan island.

“Initial reports characterized the incident as unsafe,” the Pentagon statement said.

“It must be pointed out that U.S. military planes frequently carry out reconnaissance in Chinese coastal waters, seriously endangering Chinese maritime security,” China’s Foreign Ministry spokesman Hong Lei Hong told reporters.

“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.

Speaking at a regular press briefing, he described the Pentagon statement as “not true” and said 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.

The encounter comes a week after China scrambled fighter jets as a U.S. 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 flew acrobatic manoeuvres around a U.S. spy plane.

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.

A Chinese J-11 fighter jet is seen flying near a U.S. Navy P-8 Poseidon about 215 km (135 miles) east of China's Hainan Island in this U.S. Department of Defense handout photo taken August 19, 2014. REUTERS/U.S. Navy/Handout

China claims most of the South China Sea, through which $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims.

Washington has accused Beijing of militarizing the South China Sea after creating artificial islands, while Beijing, in turn, has criticized increased U.S. naval patrols and exercises in Asia.

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

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

“DANGEROUS INTERCEPTS”

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 Center 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 U.S. 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.”

Zhang Baohui, a security expert at Hong Kong’s Lingnan University, said he believed the encounter highlighted the limitation of CUES, and shows that Chinese pilots would still fly close to U.S. surveillance planes if needed.

“Frankly, we’re always going to see these kinds of incidents as China will always put the priority on national security over something like CUES whenever it feels its interests are directly threatened,” he said.

The encounter took place in international airspace about 100 nautical miles south of mainland China and about 50 nautical miles east of Hainan island, a Pentagon spokesman said in a statement issued later on Thursday.

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.

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 9,800-foot (3,000 meter) runway.

In April 2001, an intercept of a U.S. 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 U.S. air crew members were held for 11 days until Washington apologized for the incident. That encounter soured U.S.-Chinese relations in the early days of President George W. Bush’s first administration.

Last month, the Pentagon said that Russia had intercepted a U.S. Air Force aircraft over the Baltic Sea in an “unsafe and unprofessional” way.