WASHINGTON (Reuters) - The Pentagon has concluded that an intercept of a U.S. military aircraft by Chinese fighter jets last week over the South China Sea violated an agreement the two governments signed last year, a U.S. defense official said on Thursday.

A Chinese J-11 fighter jet is seen flying near a U.S. Navy P-8 Poseidon about 215 km (135 miles) east of China's Hainan Island in this U.S. Department of Defense handout photo taken August 19, 2014. REUTERS/U.S. Navy/Handout

The Pentagon findings contradict what the Chinese Defense Ministry said earlier in the day.

Last year, the United States and China announced an agreement establishing rules of behavior 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 Organization (ICAO) standards,” U.S. Defense Department spokesman Bill Urban told Reuters.

The incident took place in international airspace last week as a U.S. military plane carried out “a routine U.S. patrol,” the Pentagon said.

Two Chinese J-11 fighter jets flew within 50 feet (15 meters) of the U.S. EP-3 aircraft, a U.S. defense 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 $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims.

Washington has accused Beijing of militarizing the South China Sea after creating artificial islands, while Beijing, in turn, has criticized increased U.S. naval patrols and exercises in Asia.

Chinese Defense 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 U.S. to stop such flights.

Urban said the two governments discussed the intercept at this weeks Military Maritime Consultative Agreement talks in Hawaii. “The United States has expressed our concern to China,” he said.

The agreement on rules of behavior for air-to-air encounters signed last year was broad in scope, addressing everything from the correct radio frequencies to use during distress calls to the wrong physical behaviors to use during crises.

Last week, Pentagon spokesman Peter Cook told reporters that it was unclear if China violated the agreement but that their actions were “unsafe.”