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Pentagon spokesman Rear Adm. John Kirby said the incident occurred Aug. 19 about 135 miles east of Hainan island in the South China Sea and involved a Navy P-8 Poseidon plane.
He said the Chinese fighter made three passes dangerously near the U.S. plane. The jet also zoomed directly in front of the Navy plane at a 90-degree angle to reveal its belly, which was packed with weaponry, he said.
At one point, Kirby said, the Chinese warplane flew alongside the Navy aircraft, putting their wingtips as close as 30 feet apart. The encounter ended with the Chinese pilot doing a barrel roll over the top of the U.S. plane.
Kirby called the incident "very, very close" and "very dangerous."
The Obama administration has lodged a protest to China through official diplomatic channels.
"This kind of behavior not only is unprofessional, it's unsafe and it's certainly not in keeping with the kind of military-to-military relations that we'd like to have with China," Kirby told reporters.
He said the U.S. had "registered our strong concerns to the Chinese about the unsafe and unprofessional intercept, which posed a risk to the safety and the well-being of the air crew and was inconsistent with customary international law."
He said he was not aware of any radio communications between the American and Chinese pilots.
The Pentagon took several days to disclose the incident because officials were reviewing the details and wanted to register the protest with Beijing, Kirby said.
The incident comes during a tense period in the South China Sea, where Beijing's territorial claims over several resource-rich shoals overlap with those of neighboring nations.
In May, China placed an oil rig in waters claimed by Vietnam, which set off a round of anti-Chinese protests and deadly riots in Vietnam. China removed the rig last month.
Ministers from the 10-member Assn. of Southeast Asian Nations, or ASEAN, issued a statement expressing concern about the growing friction with China and called for increased talks with Beijing.
The Navy has called the P-8 Poseidon "the most advanced long-range anti-submarine and anti-surface warfare aircraft in the world."
Based on a heavily modified Boeing 737, the Poseidon typically flies at elevations of 5,000 to 10,000 feet and can stay aloft for about nine hours.
The plane is outfitted with advanced radar and electronic signal sensors to identify, find and track surface targets.
It also can drop sonar systems that send back signals indicating objects below the water's surface.