US commerce secretary Don Evans has delivered a stinging rebuke to China for failing to open its markets to foreign goods.
Don Evans: Keeping up the pressure on China
Speaking during a visit to Beijing, Mr Evans said China benefited from America's commitment to free trade without reciprocating.
"China's current trade practices are exploiting our open markets and are creating an unfair advantage that is undercutting American workers," Mr Evans said.
"China needs to create an economic system that is more transparent and one that allows capital to flow freely in response to market forces."
He went on: "We have been patient, but our patience is wearing thin."
Mr Evan's comments mark a further escalation in a long-running spat between the US and China over trade policy.
US officials accuse China of keeping its currency, the yuan, artificially low against the dollar in order to boost exports to the US.
They claim this has contributed to America's burgeoning trade deficit with China, estimated at $103bn last year, and has led to the loss of thousands of US manufacturing jobs.
Sharply rising US unemployment figures ahead of next year's presidential elections have put the White House under added pressure to wring concessions from China.
However, the Chinese authorities on Monday signalled that they were in no mood to alter their currency regime.
The state-owned China Daily newspaper quoted Chinese central bank governor Zhou Xiaochuan as saying that China would stand "firm against US pressure for a revaluation."
Beijing has repeatedly rebuffed US calls for it to allow the yuan to float freely.
Mr Evans, who began an eight-day visit to China late last week, is scheduled to meet Chinese premier Wen Jiabao on Tuesday.
His visit comes days after a trip by US Trade Representative Robert Zoellick, who warned that the US may restrict China's access to its markets if Beijing did not relax its stance.