Two Chinese fighter jets have carried out an "unsafe" intercept of a US military aircraft over the South China Sea, the Pentagon says.
The incident took place on Tuesday in international airspace as the US reconnaissance aircraft carried out "a routine US patrol," a Pentagon statement said on Wednesday.
The incident comes a week after China scrambled fighter jets as a US Navy ship sailed close to a disputed reef in the South China Sea.
Another Chinese intercept took place in 2014 when a Chinese fighter pilot few acrobatic manoeuvres around a US spy plane.
The intercept is also days before President Barack Obama travels to parts of Asia from May 21-28, which will include a Group of Seven summit in Japan and his first trip to Vietnam.
The Pentagon statement said the Department of Defence was addressing the issue through military and diplomatic channels.
The Chinese embassy in Washington did not immediately respond to a request for comment.
"Over the past year, DoD has seen improvements in PRC actions, flying in a safe and professional manner," the Pentagon statement said, using an acronym for the People's Republic of China.
In 2015, the United States and China announced agreements on a military hotline and rules of behaviour to govern air-to-air encounters.
China claims most of the South China Sea, through which $5 trillion in ship-borne trade passes every year.
The Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims.
Washington has accused Beijing of militarising the South China Sea after creating artificial islands while Beijing, in turn, has criticised increased US naval patrols and exercises in Asia.
The Pentagon last month called on China to reaffirm it has no plans to deploy military aircraft in the Spratly Islands after China used a military plane to evacuate sick workers from Fiery Cross.
In April 2001, an intercept of a US spy plane by a Chinese fighter jet resulted in a collision that killed the Chinese pilot and forced the American plane to make an emergency landing at a base on China's Hainan island.
The 24 US air crew members were held for 11 days until Washington apologised for the incident. That encounter soured US-Chinese relations in the early days of President George W Bush's first administration.
Last month, the Pentagon said that Russia had intercepted a US Air Force aircraft over the Baltic Sea in an "unsafe and unprofessional" way.