NEAR MISCHIEF REEF, South China Sea — As the United States Navy reconnaissance plane banked low near Mischief Reef in the South China Sea early this month, a Chinese warning crackled on the radio.

“U.S. military aircraft,” came the challenge, delivered in English in a harsh staccato. “You have violated our China sovereignty and infringed on our security and our rights. You need to leave immediately and keep far out.”

Aboard the P-8A Poseidon maritime patrol aircraft, flying in what is widely considered to be international airspace, Lt. Dyanna Coughlin scanned a live camera feed showing the dramatic evolution of Mischief Reef.

Five years ago, this was mostly an arc of underwater atoll populated by tropical fish and turtles. Now Mischief Reef, which is off the Philippine coast but controlled by China, has been filled out and turned into a Chinese military base, complete with radar domes, shelters for surface-to-air missiles and a runway long enough for fighter jets. Six other nearby shoals have been similarly transformed by Chinese dredging.