Treasury Secretary Steven Mnuchin told CNBC on Thursday he's "closely monitoring" the weakening in the Chinese currency.

"What I've said over the last week [is] we are obviously closely monitoring the Chinese rmb and the weakening in that market," Mnuchin said in an interview with "Squawk Box."

He said the administration is looking at other currencies as well.

"The long-term strength of the dollar is important. It's the result of a very strong U.S. economy," Mnuchin continued. "But we will closely monitor, as we do in the Treasury, currency manipulation across lots of different markets. And make sure people don't use currency for unfair trade advantages."

The yuan, also called the renminbi, was at 6.77 per dollar Thursday, down about 0.15 percent. China's currency has been sliding since mid-June amid an escalating trade war between the U.S. and China. It bumped around at higher levels from February through May.

Mnuchin has said the yuan's weakness would be reviewed as part of the U.S. Treasury's semi-annual report on currency manipulation. The report is due on Oct. 15 and will be based on activity for the first six months of 2018.

President Donald Trump has accused China and the European Union of manipulating their currencies, saying it is "taking away our big competitive edge."

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—Reuters and CNBC's Patti Domm contributed to this report.