Chinese warplanes allegedly harassed a Royal Canadian Air Force (RCAF) plane “on several occasions” from April 26 to May 26 as it attempted to monitor possible sanction evasion activities by Pyongyang in international airspace near North Korea, Reuters reported on Thursday.

The Canadian Armed Forces released a statement on June 1 denouncing the recent interactions between its CP-140 Aurora long-range patrol aircraft and “several” unspecified warplanes deployed by China’s People’s Liberation Army Air Force (PLAAF).

The press release read, in part:

In these interactions, PLAAF aircraft did not adhere to international air safety norms. These interactions are unprofessional and/or put the safety of our RCAF personnel at risk. In some instances, the RCAF aircrew felt sufficiently at risk that they had to quickly modify their own flight path in order to increase separation and avoid a potential collision with the intercepting aircraft.

The PLAAF jets allegedly attempted to “divert” Canada’s patrol aircraft from its flight path during the interactions. The Canadian Armed Forces further revealed that the PLAAF’s harassment of its aircraft near North Korea between April and May was not an isolated incident, but rather indicative of behavior that has increased in frequency in recent months. Canada’s military said it had addressed these belligerent actions “through diplomatic channels” with Beijing. China’s government has yet to comment on the allegations.

The RCAF deployed a CP-140 Aurora Maritime Patrol Aircraft and supporting personnel to Kadena, Japan, from April 26 to May 26 as part of an operation known as “Op Neon.” The program is Canada’s contribution to a United Nations (U.N.)-led surveillance scheme that monitors the waters near North Korea for possible violations of international sanctions on Pyongyang.

Canada’s military describes “Op Neon” as an effort to support U.N. Security Council sanctions imposed on North Korea between 2006 and 2017 designed to “pressure North Korea to abandon its weapons of mass destruction programs and respond to North Korean nuclear weapon tests and ballistic missile launches.”

“Op NEON sees military ships, aircraft and personnel deployed to conduct surveillance operations to identify suspected maritime sanctions evasion activities, in particular ship-to-ship transfers of fuel and other commodities banned by the United Nations Security Council Resolutions,” the Canadian Armed Forces wrote on June 1.

U.N. Security Council sanctions imposed on North Korea in 2017 included “a ban on the sale of natural gas liquids to the North-East Asian nation, and on its textile exports — while also prohibiting Member States from providing work authorizations to its nationals,” according to a U.N. press release.

China, which borders North Korea, maintains loose economic ties with Pyongyang, though the two communist countries have long balanced imperfect relations. Western observers have often accused Beijing of helping North Korea skirt international sanctions by supporting illicit trade with the impoverished nation.