WASHINGTON—Treasury Secretary Jacob Lew warned ignoring recent steps by Beijing to liberalize its economy would endanger Chinese cooperation on other major geopolitical issues, particularly containing North Korea’s nuclear program.

President-elect Donald Trump has promised to declare China a currency manipulator when the Treasury Department issues its next semiannual foreign-exchange report in April, which would be the first of several steps for the administration to slap China with a slew of tariffs.

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