China's espionage operations are of "substantial concern," and the United States is taking new steps to address the threat, FBI Director Robert Mueller said Thursday.

Mueller was asked about Beijing's spying programs in the United States during a hearing of the House of Representatives Judiciary Committee, but said he could give few details in an unclassified setting.

"I can say that it is a substantial concern," Mueller said.

"China is stealing our secrets in an effort to leap ahead in terms of its military technology but also the economic capability of China.

"It is a substantial threat that we are addressing in the sense of putting -- building our program to address this threat."

China has repeatedly denied that it is spying on US military or economic intelligence, including on one occasion in March.

"We have reiterated many times that the so-called stealing of US military secrets is groundless and the accusations are due to ulterior motives," foreign ministry spokesman Qin Gang said.

Also in March, Joel Brenner, head of the US Office of the National Counterintelligence Executive, said China's intelligence services were the most active in the world in spying on the United States and were waging a "very aggressive" campaign to obtain US military secrets.