PALO ALTO, Calif.—Treasury Secretary Timothy Geithner said Monday the U.S. would not "engage" in devaluing its currency, but described China's currency as "significantly undervalued," as tensions between China and other countries remain high in advance of meetings of finance ministers and central bankers in South Korea this weekend.

Mr. Geithner's comments were among his most blunt on the Chinese currency and his sharpest retort to recent criticism from some countries that the U.S. government is pursuing policies aimed at reducing the value of the dollar to spur exports and economic growth.

"It's not going to happen in this country," Mr. Geithner said in remarks here. "The United States of America and no country in the world can devalue its way to competitiveness. It's not viable and we will not engage in it."

But Mr. Geithner said China's reluctance to unfetter its currency is "unfair to China's trading partners, including America, because it just creates a playing field that's unbalanced, provides near-term economic advantage to Chinese companies, and is damaging to what's important to everyone—that we operate in a world in which everyone operates according to rules."

He noted with approval, though, that since the beginning of September, China has allow the currency to rise somewhat faster than it did in the preceding weeks. "It has moved 3% in last couple of months," Mr. Geithner said. "We want to see that process continue."