Malaysia’s air force scrambled fighter jets to intercept 16 Chinese military aircraft off the coast of Sarawak state on the island of Borneo on Monday after they intruded into Malaysian maritime airspace and “refused to communicate with local air-traffic controllers,” Malaysian government officials said on Tuesday.

The Royal Malaysian Air Force (RMAF) issued a statement on June 1 detailing the Chinese air fleet’s incursion into Malaysia’s “maritime zone airspace,” which is the airspace over a nation’s territorial seas. International law grants coastal states complete sovereignty over their maritime zone airspace. China’s People’s Liberation Army Air Force (PLAAF) operated the intrusive fleet, which included the “Russian-made Ilyushin Il-76 and Chinese-made Xian Y-20 – transport aircraft capable of conducting multiple missions,” according to Malaysian military officials.

RMAF’s radar station in Sarawak “detected the [Chinese] aircraft on Monday at 11:53 a.m. local time, and … the aircraft were in a tactical trail formation spanning 60 kilometers, or approximately 37 miles,” Malaysia’s air force said in a statement quoted by USNI News, a website operated by the U.S. Naval Institute. “The aircraft were flying … through the Singapore Flight Information Region (FIR) before crossing into the airspace of Malaysia’s EEZ [Exclusive Economic Zone].”

“As the [Chinese] aircraft headed in the direction of Malaysia’s national airspace, the RMAF put the Hawk 208 fighter aircraft … on alert while trying to contact the PLAAF aircraft,” the statement read.

“After not receiving a response, the RMAF launched the Hawks [fighter aircraft] at 1:33 p.m. to intercept and conduct visual examination [of the Chinese PLAAF fleet],” according to the statement.

“The [PLAAF] formation, followed by the RMAF fighters, continued on past the Luconia Breakers, coming as close to as 60 nautical miles off the Sarawak coastline before turning back,” according to Malaysia’s air force.

Luconia Breakers — also known as Luconia Shoals or Beting Patinggi Ali — is one of the largest reef complexes in the South China Sea. The complex is administered by the government of Kuala Lumpur, Malaysia’s national capital, but illegally claimed by China.

Malaysia’s Ministry of Foreign Affairs announced plans on Tuesday to file a formal diplomatic protest with China over its “intrusion” into Malaysian airspace on May 31.

“Based on the report received from the Royal Malaysian Air Force, the Ministry will issue a diplomatic note of protest against the intrusion to the Government of the People’s Republic of China,” the foreign ministry said in a press release issued June 1.

“The ministry will also summon the Ambassador of the People’s Republic of China to Malaysia to provide explanation regarding this breach of the Malaysian airspace and sovereignty by the 16 PLAAF aircraft [sic],” according to the statement.

“Malaysia’s stand is clear – having friendly diplomatic relations with any countries does not mean that we will compromise our national security. Malaysia remains steadfast in defending our dignity and our sovereignty,” Malaysian Foreign Minister Hishammuddin Hussein said in the statement.

When asked by Bloomberg about China’s intrusion into Malaysian airspace on May 31 at a regular press briefing on June 2, Chinese foreign ministry spokesman Weng Wenbin replied:

To my knowledge it was a routine training conducted by China’s air force over waters to the south of Nansha Islands. It doesn’t target any country. During the training, China’s air force strictly complied with international law and didn’t enter the air space of any other country. The Chinese side has communicated with the Malaysian side over this.

Nansha Islands is the Chinese name for the disputed Spratly Island chain. The South China Sea archipelago is claimed by China, Malaysia, the Philippines, and Vietnam.