The U.S. said it will keep pressing China to strengthen a "significantly undervalued" yuan while declining to brand the world's second-largest economy a currency manipulator.

In its semi-annual report to Congress on exchange-rate policies, the Treasury Department said today that it will continue to "closely monitor" the pace of yuan appreciation and push for "policy changes that yield greater exchange-rate flexibility."

The Obama administration says China's policies keep the yuan undervalued and produce an unfair advantage in global trade. Politicians, including presumptive Republican presidential nominee Mitt Romney and Senator Charles Schumer, a New York Democrat, have complained that the administration should be more aggressive in pushing China on the currency. No country has been designated a manipulator by the U.S. since China in 1994.