Spot yuan opened at 6.6655 per dollar and fell to a low of 6.6842 per dollar at one point in morning trade, its weakest level since Aug. 16.

As of the 4:30 p.m (0830 GMT) domestic close, the yuan settled at 6.6470 per dollar.

If the yuan closes at the level in the night trading session, it would have lost 0.87 percent of its value against the dollar for the month, its first monthly loss since April and its worst monthly performance since December 2016.

China takes the official market closing price at 4:30 p.m Shanghai time (0830 GMT) into consideration when it fixes the official guidance rate, in an effort to let market forces play a bigger role in determining the yuan's value.

The market also has an evening session lasting until 11:30 p.m.

The domestic foreign exchange market will be shut for the holiday between Oct. 1 and Oct. 8. Trading will resume on Oct. 9.