The U.S. Federal Bureau of Investigation said on Wednesday that hackers it believes to be backed by the Chinese government have recently launched attacks on U.S. companies.

The "flash" warning to businesses described tools and techniques used by the hackers and asked companies to contact federal authorities if they believe they are the victims of such attacks.

The document said the agency recently obtained information regarding "a group of Chinese government affiliated cyber actors who routinely steal high-value information from U.S. commercial and government networks through cyber espionage."

FBI spokesman Josh Campbell confirmed via email that the FBI had released the document, which was obtained by Reuters and described specific mitigation steps that companies should take following attacks.

"The FBI has recently observed online intrusions that we attribute to Chinese government affiliated actors," he said. "Private sector security firms have also identified similar intrusions and have released defensive information related to those intrusions."

The Chinese embassy in Washington fired back by asking "the U.S. side to stop this kind of unfounded accusation," echoing past responses to U.S. allegations of government-backed cyber security breaches.

"I'm not aware of the investigation by the U.S. FBI," Chinese embassy spokesman Geng Shuang said. "Judging from past experience, conclusions of this kind of investigations are usually lacking in provable facts and hard evidence."