US Democratic presidential hopeful Hillary Clinton has accused China of stealing commercial secrets and government information.
She accused China of "trying to hack into everything that doesn't move in America", and urged vigilance.
US officials had named China as the chief suspect in the massive hack of the records of a US government agency earlier this year.
China had denied any involvement, and called US claims "irresponsible".
Speaking at a campaign event in New Hampshire, Ms Clinton said that China was stealing secrets from defence contractors and had taken "huge amounts of government information, all looking for an advantage."
She added that she wanted to see China's peaceful rise but that the US needed to stay "fully vigilant".
"China's military is growing very quickly, they're establishing military installations that again threaten countries we have treaties with, like the Philippines because they are building on contested property," she said.
US officials have blamed China for a major data breach of the Office of Personnel Management (OPM) that was revealed in June.
The hacking of federal government computers could have compromised the records of four million employees.
US intelligence chief James Clapper called China a "leading suspect" after the incident.
But China dismissed the accusation, saying that it was "irresponsible and unscientific".
China has previously argued that it is also the victim of hacking attacks.
Republican presidential candidates have used the recent OPM cyber hack to attack President Obama's administration, accusing it of "incompetence".
Meanwhile, Democratic candidate Martin O'Malley has called for better funding for cyber security.
The hack against the OPM is not the first time that China has been blamed for a cyber attack against the US.
An earlier attempt to breach OPM networks was blocked in March 2014, with the US saying China was behind the attack.
Previous hack attacks on US government
- In November 2014 a hack compromised files belonging to 25,000 employees of the Department of Homeland Security, as well as thousands of other federal workers
- In March 2014 hackers breached OPM networks, targeting government staff with security clearance, but the attempt was blocked before any data was stolen. The intrusion was reportedly traced to China
- In 2006, hackers believed to be based in China breached the system of a sensitive bureau in the US Department of Commerce. Hundreds of workstations had to be replaced