China's ambassador to the World Trade Organization, Sun Zhenyu, said in comments carried by the official Xinhua news agency that the United States often failed to comply with WTO rulings by citing national security concerns.

"By interpreting and applying WTO national security clauses in an excessive way, it has again seriously undermined the credibility of the multilateral trade regime," Sun was quoted as saying from Geneva.

Sun criticized the United States for recently imposing restrictions on foreign direct investment on account of apparent national security concerns.

"These have dealt heavy blows to (WTO) members' confidence in the business environment of the US," he said.

Sun's comments came as two US senators who have threatened sanctions against China for its trade policies were in China on a fact-finding mission and one month ahead of Chinese President Hu Jintao's visit to Washington.

Democrat Chuck Schumer and his Republican colleague Lindsey Graham came to Beijing after co-sponsoring a bill that would impose tariffs of 27.5 percent on Chinese imports unless China took steps to strengthen its currency.

The senators have argued that the yuan is undervalued by as much as 40 percent against the dollar, making US companies uncompetitive against Chinese rivals.

The exchange rate, the senators and other US critics contend, helped to drive up China's trade surplus with the United States to a record 202 billion dollars last year.

But Sun repeated Chinese accusations that the US restrictions on exporting high-tech products to China was a significant factor in the bilateral trade imbalance.