KUALA LUMPUR—The foreign ministry of Malaysia on Tuesday said it would summon China’s envoy to explain an “intrusion” by 16 air force planes into its airspace, after the Southeast Asian country’s military detected “suspicious” activity over the South China Sea. Malaysia’s air force said it scrambled jets on Monday to conduct visual confirmation after the planes flew within 60 nautical miles off Sarawak state of Malaysian Borneo. It described the incident as a “serious threat to national sovereignty and flight safety.” The Chinese planes did not contact regional air traffic control despite being instructed several times, the air force said. Foreign Minister Hishammuddin Hussein said Malaysia will issue a note of diplomatic protest and ask China’s ambassador to Malaysia to explain the “breach of the Malaysian airspace and sovereignty.” “Malaysia’s stand is clear—having friendly diplomatic relations with any countries does not mean that we will compromise on our national security,” …