An armed Chinese fighter jet aggressively confronted a U.S. Navy P-8 Poseidon surveillance aircraft earlier this week over international waters in the South China Sea, mounting a series of "unprofessional and unsafe" maneuvers that included passing within 20 feet of the Navy aircraft's wingtips, a Pentagon official said.

"It was very, very close and very dangerous. ... I think the message they were apparently sending is they were resisting the flight of that patrol aircraft," Rear Adm. John Kirby, a Pentagon spokesman, said Friday. The incident occurred on Aug. 19 but was officially disclosed for the first time Friday.

The Navy P-8 was on a routine mission gathering intelligence in international airspace over the contested South China Sea, about 135 miles east of Hainan Island, China's southernmost point. The incident occurred several days after Army Gen. Martin Dempsey, chairman of the Joint Chiefs, made a historic visit to nearby Vietnam.

On three separate passes, the Chinese J-11B flew directly under and alongside the Navy aircraft, at one point bringing its wingtips within about 20 feet of the P-8's wings before conducting a roll over the top of the U.S. aircraft.

The Chinese fighter jet also passed the nose of the P-8 at a 90-degree angle, showing its belly loaded with weaponry to the U.S. Navy pilot, Kirby said.

Kirby said U.S. officials contacted the Chinese government afterward and "registered our concerns through official diplomatic channels."

It was one of the most dangerous incidents between a U.S. and Chinese aircraft since the April 2001 collision of a Navy EP-3 Aeries and another Chinese fighter jet, a defense official said. That 2001 collision also occurred over the South China Sea not far from Hainan Island.

The incident is "the most recent in a rising trend of nonstandard, unprofessional and unsafe intercepts of U.S. aircraft that we have observed since the end of 2013" involving Chinese aircraft, the official said.

U.S. military officials in the Pacific region say several other similar, though somewhat less aggressive, incidents have occurred earlier this year, with all of the Chinese aircraft apparently coming from the sane Hainan-based unit.

"We are concerned that the intercepting crews from that unit are acting aggressively and demonstrating a lack of regard for the regard for the safety of our aircrews," said one defense official familiar with the incidents.