China's yuan slipped below the psychologically important level of 6.7 to the dollar Monday. That was the currency's weakest level in nearly six years.

Traders said state banks were defending the yuan firmly at 6.699 before the official close, but the yuan slipped lower to 6.7021 not long afterwards. The yuan has now lost more than 3 percent of its value against the dollar this year.

The central bank earlier this month reiterated the yuan won't depreciate in the long term. All eyes will be on the central bank's official mid-point-rate Tuesday morning.