'The contents of the advertisement of the United States are totally groundless,' said Foreign Ministry spokesman Qin Gang. 'China expresses its indignation and resolute opposition.'

According to the San Francisco Chronicle, the ad was placed in three local Chinese-language dailies - the World Journal, Ming Pao Daily and Sing Tao Daily - last week.

'Chinese living here have often helped the FBI prevent subversive elements from penetrating and harming our country. In order to protect our freedoms and democracy, we continue to seek your assistance.

'We especially welcome anyone who has information about the Chinese (government) or State Security,' the ad reportedly said.

Mr Qin said China's national security authorities followed the law and would never violate the sovereignty or territorial integrity of another nation.

'A handful of people in the United States are acting against the trend of the times and cling to a Cold War mentality, attempting to stain China's image,' he added.

The FBI action is an effort to drum up the 'China threat theory', which views China's rise as a global political, economic and military power as a threat to peace and stability, he said.

AGENCE FRANCE-PRESSE