Chinese fighter jets have reportedly flown in an “unprofessional” manner near a US Air Force aircraft flying above the Yellow Sea on Wednesday, according to a US official cited in a CNN report.

Two of China’s Su-30 jets came within 150 feet of the US Air Force’s WC-135 aircraft, nicknamed the “nuke-sniffer,” due to its ability to detect nuclear-test elements from the air. The Su-30s were reported to have flown upside down, directly above the WC-135.

NBC News confirmed from US officials that the WC-135 was operating in accordance with international law.

The latest incident would be the second disturbance between US and Chinese aircraft this year. US defense officials in February revealed an “unsafe” encounter with a Chinese surveillance plane above the South China Sea, according to CNN. During that incident, the US Navy aircraft had to change its course after the Chinese plane had come within 1,000 feet of it, increasing the possibility of a collision.

Since the encounter in February, US officials explained that these incidents between the two nations were rare. In 2016, there were two such occurrences, and in 2015, there were none.

The WC-135 has been deployed to the region in the past, due to the North Korean nuclear threat that has seen an uptick after recent missile tests. The aircraft is able to monitor nuclear weapons testing and can track radioactive activity by collecting samples of particles and chemical substances.