New York's Chuck Schumer said he would block Schwab's nomination as the next US trade representative (USTR) until he receives answers to a letter sent by himself and Republican colleague Lindsey Graham.

Their protest drew criticism that US policy risks being left rudderless at a critical time for attempts by the World Trade Organisation to liberalise world commerce.

The appointment of Schwab, 51, a trade expert with three decades of public service, was approved by the Senate finance committee late Monday. It was expected to sail through the full chamber by Friday.

But unanimous consent is required for the Senate to pass her nomination in a procedural motion without a debate or roll call vote, allowing a single senator to block the appointment.

In another letter to Schwab, Schumer said his objections were "not related in any way to your qualifications or experience".

"I simply believe that this is a very critical time for US trade relations, and I felt that your responses to several of my questions at your nomination hearing, particularly as they related to China and financial services, were unnecessarily evasive and unhelpful," he said.

Steve Norton, a spokesman at the USTR office, said aides were working on a reply to Schumer and Graham.

He said: "We are disappointed. Ambassador Schwab has very broad bipartisan support in the Senate. We hope she will be confirmed as quickly as possible."

Senators Schumer and Graham are both vocal critics of China's trade policies. They have threatened to slap a tariff of 27.5 percent on the booming country's US exports unless it reforms its currency regime.

In their joint letter to Schwab, the senators demanded to know what the USTR department intends to do to guarantee that US financial companies enjoy the same access to China that Chinese firms enjoy in the United States.

They said that in her written responses after her May 16 confirmation hearing, Schwab had stressed that the US administration was "reminding", "stressing" and "emphasizing" the importance of Chinese commitments.

"Yet it is only rhetoric to verbally push China to do what they have already promised to do," they argued.

Schwab, now a deputy USTR, was nominated by President George W. Bush last month to succeed her boss Rob Portman, who was chosen to lead the White House budget office.

Republican Senator Chuck Grassley, who chairs the finance committee, accused Schumer and Graham of playing "games" with the Schwab nomination.

"We're at a critical point in the Doha Round negotiations in the World Trade Organisation. We need Ambassador Schwab out there representing us in her full capacity as the president's trade representative," Grassley said.

"There's no reason to play games with her nomination," he said. "The senators have made their point. Delaying her confirmation doesn't add anything."

But the protesting senators said they had a serious point to make about the US administration's handling of Chinese trade questions.

"We have seen first-hand on the currency manipulation issue how limited the rhetorical strategy can be," Schumer wrote.

"China has promised time and time again to allow market forces to gradually appreciate its currency, yet the yuan has appreciated by only 1.1 percent since last July.

"We do not regard this as significant progress, and we do not want the same thing to happen with financial services liberalisation."

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