Two Chinese fighters have carried out an "unsafe" intercept of a US military aircraft over the South China Sea, the Pentagon says.
The incident happened in international airspace on 17 May as the US maritime reconnaissance aircraft carried out a routine patrol in the area, it added.
Tensions in the region have increased recently, with China and the US trading accusations over military activity.
Several nations claim territory in the resource-rich South China Sea.
A military official quoted by the Associated Press news agency said the US pilot was forced to descend about 200ft (60m) to avoid a collision.
The defence department said it was addressing the issue through military and diplomatic channels.
"Over the past year, DoD has seen improvements in PRC actions, flying in a safe and professional manner," the statement added, using the initials for People's Republic of China.
The South China Sea dispute
Washington has accused Beijing of militarising the South China Sea, which is also an important shipping route.
China in turn has criticised increased US naval patrols and exercises in Asia.
What is the South China Sea dispute?
Rival countries have wrangled over territory in the South China Sea for centuries, but tension has steadily increased in recent years.
Its islets and waters are claimed in part or in whole by Taiwan, China, Vietnam, the Philippines, Malaysia and Brunei.
China has backed its expansive claims with island-building and naval patrols, while the US says it opposes restrictions on freedom of navigation and unlawful sovereignty claims — by all sides.
The frictions have sparked concern that the area is becoming a flashpoint with possible global consequences.