In 2013, a U.S. guided-missile ship veered sharply to avoid a Chinese Navy vessel that tried to block its path in the disputed South China Sea, according to the U.S. account. The next year, the United States said a Chinese fighter jet buzzed within 9 meters (30 feet) of one of its Navy planes, in what the White House called a "deeply concerning provocation."

They are the types of risky encounters that Beijing and Washington have sought to avoid by stepping up efforts to implement a web of military communications agreements.

But the protocols in place are mostly nonbinding, contain exceptions, and at times are interpreted differently by the two sides, highlighting the risk of an unwanted escalation of tensions as the United States asserts its naval power more forcefully to counter China's maritime claims.