Hillary Clinton has accused China of stealing commercial secrets and "huge amounts" of government information.
She also claimed the country tried to "hack into everything that doesn't move in America."
Speaking at a campaign event in New Hampshire, America's Democratic presidential candidate said she wanted to see China's peaceful rise.
"But we also have to be fully vigilant," she said. "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."
US officials say China is the top suspect in the massive hacking of a US government agency that compromised the personnel records of at least 4.2 million current and former workers.
China denies hacking into the computers of the US Office of Personnel Management.
Heavy flooding and landslides caused by torrential downpours have affected nearly four million people in China, with at least 15 feared dead.
The floods, which hit southwest China's Sichuan Province, damaged homes and destroyed vehicles.
A huge landslide hit Barkam County and thousands of residents have been evacuated across the country.
A couple buried under ruins after their house collapsed in Tongnan District in Chongqing on Tuesday morning were rescued by firefighters.
More than 100,000 tonnes of frozen meat, some of which is thought to have been packaged in the 70s, has been taken off China's streets.Read the full story ›
Millions of people have signed an online petition calling for an end to China's controversial Yulin Dog Meal Festival, which begins today.Read the full story ›
Four siblings have killed themselves by drinking pesticide after they were abandoned by their parents, according to reports.Read the full story ›
Hundreds of bodies, including that of a three-year-old girl, have been retrieved from Chinese cruise ship Eastern Star pushing the death toll over 400.
Another 36 people are listed as still missing following the disaster, which is the worst in Chinese maritime history.
Only 14 people survived, including the captain, after the boat capsized on Tuesday.
Residents of China's Jinali county have begun hanging yellow ribbons around town to show their support for the victims of the Yangtze River cruise ship disaster.
At least 339 people were killed when a Chinese cruise ship carrying 456 people capsized on Monday. Only 14 people including the captain survived.
The death toll from a Chinese cruise ship that capsized during a storm in the Yangtze River has reached 396, a government spokesman has said.
Only 14 survivors, one of them the captain, were found after the ship carrying 456 people sank on Monday night.
The number of people killed when a Chinese cruise ship capsized during a storm in the Yangtze River has reached 331 according to state media, Reuters reports.
Only 14 survivors, one of them the captain, were found after the ship carrying 456 people rapidly overturned on Monday night.
Jiang Zhao, general manager of the company which operated the Eastern Star, bowed in apology for the disaster during an interview with state media on Friday, saying they would "fully" cooperate with the investigation.
Beijing has pledged there would be "no cover-up" in the probe into the tragedy.
Chinese rescue workers have fully righted a ship which capsized on the Yangtze River as officials warned more than 400 people are likely to have died in the disaster.
The Eastern Star ship, which was carrying 456 passengers, sunk in a severe storm on Monday night.
Only 14 survivors have been found so far and authorities said the chance of finding anyone else alive was "very slim".
The operation to right the ship signalled a shift in focus from finding survivors to recovering bodies.
The incident will be the country's deadliest boat disaster in decades, and families of those feared to have died have expressed anger at the response of the Chinese authorities to the disaster.