Malaysia’s Foreign Minister Hishammuddin Hussein on Tuesday said he would file a diplomatic protest note to China’s envoy, seeking an explanation for the “breach of airspace and sovereignty.” According to the Malaysian air force, the 16 Chinese military jets were spotted conducting suspicious activities over the South China Sea. On Monday, Malaysia’s air force scrambled aircraft when the planes went within 60 nautical miles (110 kilometers) of Sarawak, Malaysia’s Borneo state conduct visual confirmation. The incident was termed as a “serious threat to national sovereignty and flight safety,” according to the report.

The air force said, despite being told to do so many times, the Chinese planes failed to contact regional air traffic control. According to China’s embassy in Kuala Lumpur, the jets were doing regular flight training and “strictly abided by” international law without breaching other countries’ airspace.

A spokesperson said, China and Malaysia are friendly neighbours, and China is eager to maintain bilateral friendly discussions with Malaysia in order to safeguard regional peace and stability together. Hishammuddin has made his point clear that having a good diplomatic relationship with any country does not mean Malaysia will sacrifice its own national security.

China has been pushing a broad claim for doing unlawful activities over the South China Sea, which sees around $3 trillion in ship-borne trade pass through each year. It has also developed military facilities on artificial islands.