The air force said its radar picked up the jets flying in an “in-trail” formation near Malaysian airspace in eastern Sarawak state on Borneo on Monday.

It said the jets then headed near Malaysian-administered Luconia Shoals, a rich fishing ground in the disputed South China Sea before moving nearly 60 nautical miles (110 kilometers) off the coast of Sarawak. After attempts to engage the jets failed, the air force said it sent planes to identify them.

It found the planes were Ilyushin il-76 and Xian Y-20 strategic transporters flying at altitudes between 23,000 and 27,000 feet.

The air force said it reported the flights to the Foreign Ministry.

“This incident is a serious threat to national sovereignty and flight safety due to the air traffic density over the airways,” it said in a statement.

Foreign Ministry and Chinese Embassy officials couldn''t be reached immediately for comment.

China claims almost the entire South China Sea. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have overlapping claims. Tensions have ramped up since China began a massive land reclamation program in 2013.

The strategically important area has some of the world''s busiest sea lanes and is also rich in fisheries and may hold underground oil and natural gas reserves.

Malaysia says Chinese coast guard and navy ships intruded into its waters in the South China Sea 89 times between 2016 and 2019. Malaysia has sent six diplomatic protests to China, including one in 2017 in response to a Chinese note asserting its claim to the South Luconia Shoals.