"The so-called accusation against China on the issue of espionage is totally groundless," foreign ministry spokesman Liu Jianchao said when asked to comment on Monday's arrests.

"We urge the United States to abandon its Cold War thinking and stop groundless accusations and instead contribute to mutual trust and friendship between our two peoples."

In an indirect reference to the United States, Liu accused "certain countries" of repeatedly making "complete fabrications" about Chinese spy activities.

"We're very tired of this," he said.

A Pentagon official was among four people charged on Monday with espionage activities involving sensitive military and aerospace secrets.

The US official, Gregg William Bergersen, is a weapons systems policy analyst at the Pentagon's Defense Security Cooperation Agency, which implements the US Defense Department's foreign military sales programme.

One of the other men charged is Taiwan-born US citizen Tai Shen Kuo, who stands accused of having worked under the direction of an unnamed Chinese official to obtain classified information from Bergersen.

Chinese citizen Yu Xin Kang was the other charged over leaking of the military and aerospace secrets.

In another case, former Boeing engineer Dongfan "Greg" Chung, a China-born US citizen, was charged with stealing and turning over trade secrets to Beijing, including the space shuttle used for US human space flight missions.

In announcing the arrests, Assistant US Attorney General Kenneth Wainstein said both cases aimed "to get a hold of our nation's military secrets."

"Such espionage networks pose a grave danger to our national security and to our economic position in the world," he said.

The arrests followed repeated charges by the United States that China is ramping up its espionage activities as it becomes an increasingly wealthy and powerful nation.

In July last year, FBI Director Robert Mueller told Congress that China's espionage operations were a "substantial concern" and Beijing was stealing US secrets to boost its fast-developing military and economy.

And in September, the US National Intelligence director, Michael McConnell, said Chinese and Russian spies were stalking the United States at levels close to those seen during the tense covert espionage duels of the Cold War.

"China and Russia's foreign intelligence services are among the most aggressive in collecting against sensitive and protected US systems, facilities and developmental projects," McConnell said.

In other recent arrests, a Chinese national and a US citizen were in September charged in the United States with conspiring to steal sensitive microchip designs capable of use in military technology.

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