SHANGHAI: China's yuan weakened slightly against the US dollar on Wednesday and looked set for its fourth straight day of losses, after the central bank guided its official yuan fixing to the lowest level in a week.

Prior to the market open, the People's Bank of China (PBOC) set the midpoint rate at 6.6779 per dollar, 90 pips or 0.13 percent softer than the previous fix of 6.6689.

Wednesday's guidance was the weakest since Aug.9.

Traders said the fixing largely matched their expectations and reflected gains in the dollar overnight as upbeat US retail sales data kept the chance of another interest rate hike this year alive.

In the spot market, the onshore yuan opened at 6.6870 per dollar and was changing hands at 6.6876 at midday, 16 pips weaker than the previous late session close and 0.15 percent softer than the midpoint.

Its offshore counterpart weakened early and briefly breached the psychologically important 6.7 per dollar level. It was trading at 0.17 percent lower than the onshore spot at 6.6988 of midday.

One trader at a Chinese bank said market participants in the onshore market were not so eager to test the 6.7 level.

Some pointed out that dollar demand by oil firms may rise soon to pile pressure as these firms usually build up their dollar reserves mid-month for their business needs. And some other companies' dollar purchases for trade and financing demands may show up later in the month.

Official data showed on Wednesday that net foreign exchange sales by China's central bank fell to the lowest level last month since October 2015, confirming market views that pressure from capital outflows has eased due to tighter government curbs and a rebound in the yuan in recent months.

The PBOC sold a net 4.6 billion yuan ($687.90 million) worth of foreign exchange in July, down from 34.3 billion yuan in June, according to Reuters calculations based on central bank data.

The Thomson Reuters/HKEX Global CNH index, which tracks the offshore yuan against a basket of currencies on a daily basis, stood at 94.8, weaker than the previous day's 94.82.

The global dollar index fell to 93.825 from the previous close of 93.853.

Offshore one-year non-deliverable forwards contracts (NDFs), considered the best available proxy for forward-looking market expectations of the yuan's value, traded at 6.836, 2.31 percent weaker than the midpoint.

One-year NDFs are settled against the midpoint, not the spot rate.