WASHINGTON: Two US senators on Tuesday joined a growing chorus of calls in the Congress to punish China for allegedly keeping its currency artificially low in an “unlawful” move that hurts US exports.

Democratic Senator Sherrod Brown and Republican Olympia Snowe told US Treasury Secretary Timothy Geithner that they would seek to have US authorities treat currency undervaluation as an improper subsidy for Chinese goods.

“In the coming weeks, we intend to introduce legislation in the Senate to address the unlawful practice of currency manipulation and to penalize violators of our global trade rules,” they wrote in a letter to Geithner.

The proposal, which could lead to countervailing duties on Chinese exports, would mirror a bill that cleared the US House of Representatives by a 348-79 margin in September but stalled in the Senate.

With China's President Hu Jintao due in the United States Tuesday for a delicate state visit, US lawmakers have stepped up charges that the yuan is undervalued in what amounts to a competitive advantage for Beijing's exports.

Sherrod and Snowe made their letter public one day after three Democratic senators vowed to unveil legislation to make it harder for the US Treasury Department not to formally cite China as a currency manipulator.

“Our message to President Hu is, 'Welcome to America, but we want to make sure we have a fair trading system,'” said Michigan Senator Debbie Stabenow, who joined fellow Democrats Charles Schumer and Bob Casey in the announcement.

The measure would also impose stiff new penalties on designated countries, including tariffs on the countries' exports and a ban on any companies from those countries receiving US government contracts, they said in a joint statement.

Although similar measures have fallen short in recent years, Schumer said, “We think we have majority support in the House and Senate... and we think this will pass.”