Beijing: Two Chinese fighter jets flew too fast and too close to a US military aircraft patrolling the East China Sea, prompting a formal protest to the Chinese government, the US Air Force said on Friday.

The incident on Wednesday involved a US WC-135 Constant Phoenix aircraft and two Chinese SU-30 jets that both flew in an “unprofessional” and dangerously close way, according to a spokeswoman for the Pacific Air Forces, Lt. Col. Lori Hodge.

The US 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.

The US newspaper Stars and Stripes reported last month that a WC-135 had been deployed to the US 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 US missile experts.

The US protest over the military manoeuvre by the Chinese jets on Wednesday appeared to be the first since President Donald Trump took office.

After his meeting with China’s leader, Xi Jinping, last month, Trump stressed that he believed China would exert extra pressure on North Korea, its ally, to change course and modify its nuclear weapons programme.

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 DPRK,” 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 US aircraft may have been prompted by concerns that the US 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,” Shi said.

It was also possible, he said, that China had decided to enforce the parameters of the “air defence identification zone” that Beijing declared over the East China Sea in 2013. The US has refused to recognise 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 defence treaty with Japan, a stance affirmed by Vice President Mike Pence during his recent visit to Tokyo.