According to BBC, cyber-defence company Mandiant told the New York Times daily that the army's Unit 61398, believed to operate from a heavily guarded building in Shanghai, has recently stepped up its activity.
The company did not name which US agencies and businesses were attacked.
In February, the hacking report was denied by the Chinese government.
Earlier, Mandiant published detailed evidence suggesting that Unit 61398 was behind the vast majority of attacks on American organisations, BBC said.
Government documents, intellectual property, blueprints and many other confidential papers were hacked, the firm said.
However, following condemnation by the US administration, the Chinese unit apparently scaled back its activity, uninstalling spying tools and the remote access code it had placed on networks, the report said.
But now, the unit has sprung back into action and was working at about 70 percent of its former capacity, Mandiant said.
In a bid to hide its tracks, the unit has started using different computers to insert its remote access tools, the company said.
China has persistently denied sponsoring the attacks, saying the US is the real cyber-aggressor.
It said Mandiant's earlier report was flawed and did not contain enough proof to back up its accusations.
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