Chinese naval officials warned a United States surveillance aircraft eight times Wednesday to vacate the area around a series of manmade islands that China has built near contested territory in the South China Sea, CNN reports. The surveillance plane conducted a scouting mission to assess China’s progress in building military installations on the islands, a practice Beijing undertook despite the U.S. government’s objections.

China has created at least three artificial islands in the South China Sea to press its claim on the Spratlys, a chain of natural islands in the South China Sea that China, the Philippines, Malaysia, Taiwan, Vietnam and Brunei all wish to control, Newsweek reports. The U.S. aircraft was performing reconnaissance on China’s rapidly expanding artificial outposts when the Chinese navy made contact.

“This is the Chinese navy … This is the Chinese navy … Please go away … to avoid misunderstanding,” a Chinese naval official communicated to the plane in English, according to a CNN reporter who witnessed the exchange. The surveillance plane’s pilot responded that the aircraft was operating in international waters, a statement in line with the U.S. government’s official stance on the Spratlys, according to the Wall Street Journal.

China began construction on the artificial islands in 2013, using special equipment to repurpose sand at the bottom of the area’s 300-foot-deep waters to expand land at the surface. Chinese workers have added 2,000 acres of land to the artificial islands in the past two years, CNN notes. The U.S. surveillance plane obtained footage this week that showed one of the islands, called Fiery Cross Reef, now has a military barracks and a massive runway. The Chinese navy and coast guard maintains a strong presence around the islands.

“I’m scratching my head like everyone else as to what’s the [Chinese] end game here. We have seen increased activity even recently on what appears to be the building of military infrastructure,” Capt. Mike Parker, who commands U.S. surveillance aircraft in Asia, told CNN.