US Defence Secretary Chuck Hagel has accused the Chinese government and military of cyber espionage.
United States officials have made similar comments before, but Mr Hagel was speaking at a security conference in Singapore in front of Chinese military officials.
The BBC reports officials from both countries talk frequently of the threat of cyber attacks and accuse each other of involvement. They also stress that the US and China must work together.
Mr Hagel's speech followed the same pattern. After chiding China, he said:
"We are determined to work more vigorously with China and other partners to establish international norms of responsible behaviour in cyberspace."
Earlier this week US media published details of a leaked Pentagon report that claimed Chinese hackers had accessed secret US weapons programmes.
The report said: "China is using its computer network exploitation capability to support intelligence collection against the US diplomatic, economic, and defence industrial base sectors."