The United States Air Force announced on Friday that two Chinese fighter jets flew too close to a United States plane patrolling the East China Sea. The air force went so far as to complain to the Chinese government. China denied the allegations that the two Chinese fighter jets acted unprofessionally in shadowing the WC-135, an American military plane, saying that its aircraft were acting in accordance with the law.”The WC-135 was operating in accordance with international law. While we are still investigating the incident, initial reports from the US aircrew characterized the intercept as unprofessional,” the Air Force said.

The WC-135 is a so-called “sniffer plane” designed to scan the atmosphere for signs of nuclear activity.  Related remarks from the US side are inconsistent with fact,” the Chinese Ministry of Defense said in a statement posted to its website late Friday.

“On May 17, a US reconnaissance aircraft was carrying out an operation in airspace over the Chinese Yellow Sea (the northern part of the East China Sea), and Chinese aircraft acted to identify and investigate in accordance with the law,” the statement said, calling the action “professional” and “safe”.

The statement out of Beijing came as Chinese State Councilor Yang Jiechi discussed preparations for the first round of the China-U.S. diplomatic and security dialogue in a phone conversation Saturday with US Secretary of State Rex Tillerson.

China’s Ministry of Foreign Affairs noted in a statement that Yang and Tillerson “exchanged views… of common concern such as the Korean Peninsula,” but did not say whether the aircraft incident was discussed.Mid-air interceptions occur routinely in international airspace, but the US military will often call out foreign pilots if it judges the manoeuvres to be risky or unprofessional.Meanwhile, China urged the US to halt such reconnaissance exercises in order to prevent future incidents.”The American military’s frequent reconnaissance operations are the root cause of security issues between the Chinese and US navy and air forces,” the Chinese defence ministry’s statement said.The US Air Force said in an earlier statement Friday that its plane, a WC-135 Constant Phoenix, was conducting a “routine mission” in international airspace when it was intercepted by two Chinese Sukhoi SU-30 fighter aircraft.The United States has complained to China through diplomatic and military channels, she said in a statement. The “speeds and proximity” of the two Chinese planes, coupled with the “maneuvers” of one of the pilots, raised the concerns, she said.The WC-135, a modified Boeing C-135, is designed to detect radioactive debris after the detonation of a nuclear device and is informally known as a “sniffer.” The Chinese Ministry of National Defense did not reply to request for comment.

During the Obama administration, the United States and China established rules of engagement for air-to-air encounters that were supposed to lead to discussions between the two militaries in the event of an episode like the one on Wednesday.

The American newspaper Stars and Stripes reported last month that a WC-135 had been deployed to the United States military base at Okinawa, Japan, in preparation for a possible sixth nuclear test by North Korea. Such a test has not taken place, although North Korea on Sunday launched an intermediate-range ballistic missile in a test that was described as successful by American missile experts.The American protest over the military maneuver by the Chinese jets on Wednesday appeared to be the first since President Trump took office.After his meeting with China’s leader, Xi Jinping, last month, Mr. Trump stressed that he believed China would exert extra pressure on North Korea, its ally, to change course and modify its nuclear weapons program.

But China has indicated that it has done all that it can and that it is now up to the United States to accept China’s advice and negotiate with North Korea. “Pressure must be coupled with direct talks with the D.P.R.K.,” the Chinese ambassador to the United States, Cui Tiankai, wrote in an essay in USA Today this month, using the initials for the Democratic People’s Republic of Korea, the North’s official name.The maneuvers by the Chinese jets near the American aircraft may have been prompted by concerns that the United States was spying on Chinese military hardware and bases inside China, said Shi Yinhong, a professor of international relations at Renmin University of China in Beijing.The deployment of the aircraft ostensibly to search for evidence of a North Korean nuclear test may have been considered by the Chinese as an excuse for spying, he said.

“Maybe the Chinese side suspects the reconnaissance plane not only wanted to know about North Korea, but wanted to know what’s going on in China with advanced military equipment,” Mr. Shi said.

It was also possible, he said, that China had decided to enforce the parameters of the “air defense identification zone” that Beijing declared over the East China Sea in 2013. The United States has refused to recognize the zone, which includes the airspace over disputed islands that both China and Japan claim to be theirs. The islands are called the Senkaku by Japan and the Diaoyu by China.

China has demanded that foreign aircraft operating within the zone declare their whereabouts. In response, Washington has said that the Senkaku fall under the jurisdiction of the United States’ mutual defense treaty with Japan, a stance affirmed by Vice President Mike Pence during his recent visit to Tokyo.

The Chinese may be taking advantage of what they sense to be Mr. Trump’s weakening political position, Mr. Shi said.

“Trump, because of his domestic political weakness, does not want to have trouble with China,” he said. “This may be to show that Trump dare not harm the China relationship.”

Colonel Hodge said that United States military aircraft routinely flew through international airspace throughout the Asia-Pacific region and that the flight over the East China Sea was no exception.

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