During a Fourth of July campaign organising event at a house party in Glen in northern New Hampshire yesterday, Clinton said 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," the 67-year-old former secretary of state said in a strongly worded speech.
"They're also trying to hack into everything that doesn't move in America. Stealing commercial secrets...from defence contractors, stealing huge amounts of government information, all looking for an advantage," said Clinton, the frontrunner to win the Democratic nomination for the presidential polls.
Hackers believed to be working for the Chinese military breached US government computers from nearly every agency in recent months, possibly compromising the personal data of 4 million current and former employees.
US officials blamed China for the data breach and multiple media outlets said that investigators believe that it was done by the Chinese military.
China, however, denied any involvement, and called US claims "irresponsible".
Clinton also said that she remains hopeful the US can reach a "strong verifiable deal" to curb Iran's nuclear weapons programme by next week's deadline.
"I'm hoping it's a strong, verifiable deal that will put the lid on Iran's nuclear weapons ambitions," Clinton said. "Even if we are successful, however, Iran's aggressiveness will not end."
"Just because we get the nuclear deal, if we can get it done, doesn't mean we're going to be able to be breathing a big sigh of relief," Clinton said.
Her remarks comes as representatives from Iran and the P5+1 group - Britain, China, France, Germany, Russia and the US -- are involved in negotiations to clinch a nuclear deal.
The US and Iran have no diplomatic ties since the 1979 storming of the American embassy in Tehran and the 444-day hostage crisis that ensued.
Clinton also said that Russian President Vladimir Putin's moves to expand Russia's boundaries, such as the annexation of Crimea, posed a challenge for the US.