WASHINGTON (AFP) - The United States expressed concern on Friday over reports China has used a powerful censorship tool dubbed "Great Cannon" to attack websites around the world.

Researchers reported in April that the "Great Cannon" is an online attack system used to hijack web traffic and enforce the country's broad censorship of information online.

The system was used to shut down websites aimed at helping Chinese bypass the country's extensive online restrictions known as the "Great Firewall," experts said.

"We are concerned by reports that China has used a new cyber capability to interfere with the ability of worldwide internet users to access content hosted outside of China," State Department spokesman Jeff Rathke said following a question about the program.

Rathke said the cyber attack manipulated Chinese web traffic and "and turned it into malicious traffic directed at US sites."

"We have asked Chinese authorities to investigate this activity and provide us with the results of their investigation," he said.

Experts at the University of Toronto reported on the Great Cannon last month, noting denial of service attacks carried out by the system.

The report supported claims by an activist organization which said China was seeking to shut down its online service that offer ways to access content from blocked websites.

Great Cannon gives China cyberattack capabilities similar to the US National Security Agency's Quantum program, revealed in documents leaked by former NSA contractor Edward Snowden, experts said.