The US military’s Pacific headquarters in Honolulu says a Chinese fighter jet flew dangerously close to a US Air Force (USAF) reconnaissance aircraft over the South China Sea.

US Indo-Pacific Command (INDOPACOM) on 29 December said a Shenyang J-11 fighter from the People’s Liberation Army Navy (PLAN), the official name of the Chinese navy, conducted an “unsafe intercept” of an American Boeing RC-135 Rivet Joint jet on 21 December.

“The PLAN pilot flew an unsafe manoeuvre by flying in front of and within 20ft [6m] of the nose of the RC-135, forcing the RC-135 to take evasive manoeuvres to avoid a collision,” INDOPACOM says.

The Hawaii-based command, which oversees operations from the west coast of North America to the Indian subcontinent, adds its RC-135 was “lawfully conducting routine operations over the South China Sea in international airspace”.

Mobile phone video of the 21 December incident released by INDOPACOM shows a single J-11 armed with four wing-mounted missiles and the light blue livery common on many Chinese military aircraft.

The J-11 comes alongside the RC-135, which quickly drops altitude and pulls away to the right.

The USAF describes the RC-135 platform as a Boeing C-135 Stratolifter transport “heavily modified” by L3Harris to provide “near real time on-scene intelligence collection, analysis and dissemination capabilities”.

The type’s onboard sensor package allows the crew to “detect, identify and geolocate signals throughout the electromagnetic spectrum”, the service adds.

The day of the intercept marked the start of a week-long joint naval exercise between China and Russia in the East China Sea, according to the Chinese Ministry for National Defense (MND). However, it is unclear if the exercise was at all related to the RC-135 intercept, as the MND says the drills were taking place off the coast of Zhejiang Province, several hundred miles northeast of the South China Sea.

Beijing claims much of the South China Sea as its sovereign territory, citing loosely defined and centuries-old historic rights around several groups of islands and submerged reefs.

Legal analysis of Beijing’s South China Sea territorial claims by the US Department of State found them to “have no basis in international law”.

Watch video of the PLAN J-11 intercepting a USAF RC-135 here: