The Pentagon has concluded that an intercept of a US military aircraft by Chinese fighter jets last week over the South China Sea violated an agreement the two governments signed last year.
The Pentagon findings contradict what the Chinese defence ministry said earlier in the day.
Last year, the US and China announced an agreement establishing rules of behaviour to govern air-to-air encounters and creating a military hotline.
"The review of the Chinese intercept of one of our reconnaissance aircraft has assessed the intercept to have been unsafe based upon the Memorandum of Understanding with China and International Civil Aviation Organisation standards," US Defense Department spokesman Bill Urban told Reuters on Thursday.
The incident took place in international airspace last week as a US military plane carried out "a routine US patrol", the Pentagon said.
Two Chinese J-11 fighter jets flew within 15 metres of the US EP-3 aircraft, a US defence official said at the time. The official said the incident took place east of Hainan Island.
The incident came at a time of heightened Sino-American tensions in the South China Sea. China claims most of the area, through which US$5 trillion (NZ$7.41 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.
Chinese defence ministry spokesman Yang Yujun told a news briefing on Thursday that China's aircraft acted completely professionally and in line with an agreement reached between the countries on rules governing such encounters.
However, he said the agreement could only provide a "technical standard", and the best way of resolving the problem was for the US to stop such flights.
Mr Urban said the two governments discussed the intercept at the Military Maritime Consultative Agreement talks in Hawaii this week.
"The United States has expressed our concern to China," he said.
Last week, Pentagon spokesman Peter Cook told reporters that it was unclear if China violated the agreement but that their actions were "unsafe".