The 12-storey building, which houses the unit, sits in an unassuming residential area and is surrounded by a wall adorned with military propaganda photos and slogans; outside the gate a sign warns members of the public they are in a restricted military area and should not take pictures.

There were no obvious signs of extra security on Tuesday.

The Chinese Foreign Ministry said the government firmly opposed hacking, adding that it doubted the evidence provided in the U.S. security group’s report.

“Hacking attacks are transnational and anonymous. Determining their origins are extremely difficult. We don’t know how the evidence in this so-called report can be tenable,” spokesman Hong Lei told a daily news briefing.

“Arbitrary criticism based on rudimentary data is irresponsible, unprofessional and not helpful in resolving the issue.”

Hong cited a Chinese study which pointed to the United States as being behind hacking in China.

“Of the above mentioned Internet hacking attacks, attacks originating from the United States rank first.”

“ECONOMIC CYBER ESPIONAGE”

Some experts said they doubted Chinese government denials.