By Andrea Shalal

NEW YORK (Reuters) - U.S. Treasury Secretary Janet Yellen said on Wednesday U.S. President-elect Donald Trump's plan to set up a new government agency to collect tariffs would duplicate an existing agency and was unlikely to save money.

Yellen, taping an appearance on "The Late Show with Stephen Colbert," dismissed Trump's plan for an "External Revenue Service," first announced on Tuesday on his social media platform Truth Social.

"If they're looking to save money for American taxpayers, setting up a duplicative agency doesn't seem like a good first step," she told the U.S. television comedian.

Trump on Tuesday said he would create the new agency on Jan. 20, the day he takes office, "to collect tariffs, duties, and all revenue" from foreign sources.

He did not specify if the new agency would replace collections of tariffs, duties, fees and fines by the existing U.S. Customs and Border Protection, or the collection of taxes on foreign corporate and individual income by the Internal Revenue Service.

It was unclear whether the move would create additional government bureaucracy, which would appear to go against the plans of Trump's informal Department of Government Efficiency, an effort led by billionaire Elon Musk and former biotech executive Vivek Ramaswamy aimed at finding trillions of dollars in budget savings by streamlining government operations.

Yellen also took aim at Trump's repeated promises to impose new tariffs, saying they would amount to a "tax increase for the American consumer."

Trump has proposed a 10% tariff on global imports, a 25% punitive duty on imports from Canada and Mexico until they clamp down on drugs and migrants crossing borders into the U.S., and a 60% tariff on Chinese goods.

Trade experts say the duties would upend trade flows, raise costs and draw retaliation against U.S. exports.

Yellen said U.S. consumers would face higher costs for any imported goods and tariffs would make U.S. companies less competitive globally, while failing to address Americans' concerns about higher prices.

"What they're going to see is the cost of making goods and services is going to go up. They're going to be less competitive in the global economy," she said. "So this doesn't seem like a way to address the things that Americans have said are bothering them."