The Pentagon on Friday called a Chinese jet’s encounter with a U.S. anti-submarine warfare aircraft an "aggressive" and "dangerous" act and said it has protested the action with Beijing.

Pentagon Press Secretary Rear Adm. John Kirby told reporters that the incident took place Tuesday in international airspace.

"We have 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," Kirby said, adding that the incident was "very, very close, very dangerous."

"Also—and we’ve made this clear—that it undermines efforts to continue developing military-to-military relations with the Chinese military."

Pentagon spokesman Lt. Col. Jeff Pool said the aerial incident took place 135 miles east of Hainan Island when a Chinese J-11, a version of the Russian Su-27, came within 20 feet of a U.S. Navy P-8 anti-submarine warfare aircraft.

"The intercept was aggressive and demonstrated a lack of due regard for the safety and well-being of the U.S. and Chinese aircrews and aircraft," Pool said in a statement, noting it was one of the most dangerous aerial encounters with the Chinese since the April 2001 EP-3 mid-air collision with a Chinese J-8.

Pool called the encounter with the armed Chinese fighter "a dangerous intercept of a U.S. Navy P-8 Poseidon that was on a routine mission."

"On three different occasions, the Chinese J-11 crossed directly under the U.S. aircraft with one pass having only 50 to 100 feet separation between the two aircraft," the spokesman said.

"The Chinese jet also passed the nose of the P-8 at 90 degrees with its belly toward the P-8 to show its weapons loadout," he added.

"In doing so, the pilot was unable to see the P-8, further increasing the potential for a collision," Pool said. "The Chinese pilot then flew directly under and alongside the P-8 bringing their wingtips within 20 feet and then before he stabilized his fighter he conducted a roll over the P-8 passing within 45 feet."

According to the Pentagon, the latest encounter is part of a rising trend of "nonstandard, unprofessional and unsafe intercepts of US aircraft" that began in late 2013.

The Chinese jet originated from the same PLA air force unit in Hainan that was responsible for other close intercepts in March, April and May, Pool said.

"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," he said. "We have raised our concerns over this unsafe behavior to the PRC."

At Martha’s Vineyard, where President Obama is vacationing, Deputy White House Deputy National Security Adviser Ben Rhodes also criticized the Chinese for the incident that he described as a "provocation."

"It’s obviously a deeply concerning provocation, and we have communicated directly to the Chinese government our objection to this type of action," Rhodes said.

The incident could further complicate efforts to develop closer military relations. "What we’ve encouraged is constructive military-to-military ties with China, and this type of action clearly violates the spirit of that engagement, and we’ve made our concerns known directly to Beijing," Rhodes said.

Defense officials said the latest encounter highlights China’s continued aggressiveness in the region.

The P-8, a new, militarized Boeing-737 anti-submarine warfare aircraft, was conducting routine surveillance of the Chinese coast over the South China Sea, not the East China Sea as initially reported by the Free Beacon.

Other defense officials said the Chinese Su-27 interceptor carried out a barrel roll over the top of the aircraft—a move described by officials as dangerous and meant to threaten the surveillance aircraft.

It was the second threatening encounter of a U.S. surveillance aircraft this year. In April, a Russian Su-27 flew within 100 feet of a U.S. Air Force RC-135 aircraft during another dangerous intercept over waters north of Japan.

One defense official said the Pentagon’s failure to produce a tough response to the April event likely spurred the Chinese to conduct the similar threatening intercept on Monday.

Chinese military officials have said they oppose all U.S. electronic surveillance flights and described ship-based monitoring of their facilities and territory an encroachment of sovereignty. U.S. military officials have said the monitoring is carried within international airspace and thus does not violate international or Chinese law.

The Chinese attempt at aerial intimidation comes amid unprecedented Chinese military exercises held recently and currently underway in the Yellow Sea, East China Sea, and South China Sea.

On Monday, Chinese air force and navy jets conducted combat simulation drills in the East China Sea—a possible target of the P-8’s monitoring.

China also is holding international military exercises in Inner Mongolia with Russia and several Central Asia states that are part of the Beijing-led anti-U.S. alliance known as the Shanghai Cooperation Organization.

The P-8 that was intercepted by the Su-27 is part of the Navy’s first squadron of new sub hunters deployed to Asia. Six P-8s, that can fire both missiles and torpedoes, are under the command Navy’s Seventh Fleet and are based at Okinawa’s Kadena Air Base. They support the fleet’s maritime surveillance operations as part of the U.S. pivot to Asia.

The P-8s were deployed in December—a month after China declared an air defense identification zone over the East China that encroaches on both Japanese and South Korean maritime zones. The U.S. government said it does not recognize the Chinese defense zone. China has threatened to use force to maintain its control over the area covering most of the East China Sea.

The Navy has described the P-8 as "the most advanced long range anti-submarine and anti-surface warfare aircraft in the world." The jet also conducts maritime intelligence, surveillance, and reconnaissance (ISR) missions.

The U.S.-China close encounter also is a setback for Adm. Samuel Locklear, commander of the U.S. Pacific Command, who has been leading Obama administration efforts to develop closer relations with the Chinese military.

Locklear has sought to play down the growing military threat from China as part of efforts to develop closer cooperation with the Chinese military.

The commander’s dovish policies are being opposed by some in the Pentagon and Air Force who are concerned that the conciliatory approach will appease the Chinese at a time when Beijing has made aggressive territorial claims in the East China Sea and South China Seas.

Rick Fisher, a China military affairs analyst, said increased U.S. surveillance flights near China are part of the United States’ strategy of responding to China’s aggressive imposition of controls in disputed maritime regions.

"In response, China is applying the same aggressive flying intimidation tactics to U.S. surveillance aircraft that it is using on Japanese surveillance aircraft," said Fisher, a senior fellow with the International Assessment and Strategy Center.

Chinese warplanes conducted similar close intercepts to Japanese P-3 aircraft in May and June, flying within 50 feet of the aircraft, Fisher said.

"The U.S. needs to consider a stronger response and make clear to China that unprovoked deadly aggression will result in an allied military response," Fisher said.

The latest Chinese aerial assertiveness should prompt the United States to conduct mount joint fighter escorts with Japan’s military for surveillance aircraft, he said. Additionally, the Pentagon should increase the number of U.S. fighters deployed to Okinawa, and to request that the Philippines permit the stationing of a wing of fighters at Philippine air bases, as well as boost U.S. military assistance to the Manila government.

Fisher said the Chinese objective with the aggressive aerial encounters is to "make U.S. political leaders fear another ‘April 1’ incident."

In April 2001, a Chinese F-8 interceptor crashed into a U.S. EP-3 surveillance aircraft off the southern China coast, causing the J-8 to crash and nearly causing the crash of the EP-3.

That encounter set off an international crisis after the propeller-driven U.S. aircraft made an emergency landing on China’s Hainan Island and the 24 crew members were imprisoned for 10 days.

"This kind of intimidation is intended to make White House officials fear a larger incident with China and to ‘stand down’ American surveillance flights," Fisher said. "Beijing is hoping to take advantage of the distraction of these U.S. officials by multiple crises in Iraq and the Ukraine to push the Americans out of maritime regions in Asia that China is seeking to dominate."

Until Monday’s encounter, China had been operating its intercepts in a more careful manner, defense officials said, describing most past encounters as "professional."

The U.S. military has sought to engage China in talks on maritime rules of engagement and a code of conduct aimed at preventing such close encounters with limited success.

In the RC-135 encounter, the U.S. electronic surveillance aircraft was flying near the Russian Far East coast north of Japan on April 23 when an the Russian Su-27 intercepted the jet.

During that encounter, the Russian warplane rolled sideways to reveal its air-to-air missiles and then flew within 100 feet of the RC-135 cockpit. The incident was video recorded by the crew but the Pentagon declined to release the video.

The Pentagon protested the Russian encounter with officials in Moscow. However, no additional steps were taken to warn the Russians about further dangerous intercepts.

Fisher said U.S. P-8s have flown surveillance missions over the South China Sea, where China has been engaged in aggressive naval and coast guard tactics against Vietnam and Philippines over competing maritime claims.

"If such patrols are over shallower waters near to China, another ‘controlled crash’ into the P-8 could also be part of a Chinese intelligence operation to capture the latest U.S. Navy anti-submarine and patrol aircraft," he said.

"China is just now testing its first long range anti-submarine aircraft based on the turboprop powered Y-9 transport," he added. "Gaining insights into the twin-turbofan powered P-8 may accelerate a likely Chinese program to make an ASW/maritime patrol version of its twin-turbofan C-919 regional airliner."

Update: This story and headlines have been updated with a statement from the Pentagon.