SHANGHAI (Reuters) - China's yuan fell for a seventh straight session on Friday, and was on course for its third weekly loss in four as the United States prepared to ratchet up tariff pressure next month.

The yuan has weakened to fresh 11-1/2 year lows on fading hopes of a U.S-China trade deal anytime soon, although the Trump administration says face-to-face talks are still expected in September.

The yuan has now depreciated around 2.8% to the dollar since President Donald Trump said on Aug. 1 he would impose tariffs on another $300 billion of Chinese goods. Days later, China let the yuan slip through the closely watched and long supported 7-per-dollar level, and Washington labelled it a currency manipulator.

However, traders said China's central bank may have signalled an intention to stabilise the currency on Friday through its daily official midpoint setting. Friday's fall of 0.15% was not as sharp as the 0.34% slide in the previous session.

Prior to the market opening, the People's Bank of China (PBOC)lowered its official yuan midpoint to a fresh 11-year low of 7.0572 per dollar, 82 pips weaker than the previous fix of 7.0490.

Though it was the weakest guidance rate since March 21, 2008, it was stronger than the 7.0674 level market watchers had expected. Traders took that as an official attempt to slow the pace of yuan declines.

"The official fixing didn't match our forecast, (the central bank) must have heavily used its counter-cyclical factor," said a trader at a Chinese bank in Shanghai.

China first introduced the unspecified counter-cyclical factor into its midpoint fixing formula in May 2017 in what traders believed was a move to reduce price swings and counteract expectations of further yuan depreciation.

The central bank was believed to have heavily used the X factor earlier this month as authorities sought to slow the currency's decline after letting it breach the key 7 to the dollar level on Aug. 5.

In the spot market, onshore yuan opened at 7.0920 per dollar and was changing hands at 7.0937 at midday, 107 pips weaker than the previous late session close.

If the yuan finishes the late night session at the midday level, it would have lost 0.73% to the dollar for the week.

Many market participants said they consider 7.1 as China's floor for the onshore yuan for now, although the offshore yuan briefly crossed that level on Friday and weakened to 7.1382 at one point early this month.

"Corporate dollar buying remained strong today," said a second trader at a Chinese bank.

Unlike earlier in the week, traders said they had not yet seen state-controlled banks stepping into the market on Friday.

Major banks were seen receiving dollar liquidity in the forwards market before selling the greenback in the onshore spot market earlier this week, sources told Reuters. And big banks were also seen selling dollars at around 7.07 per dollar on Thursday morning.

In global markets, the greenback was steady in the morning on expectations U.S. Federal Reserve Chairman Jerome Powell would stick with his message that the central bank has not entered a prolonged monetary easing cycle.

The global dollar index rose to 98.329 at midday from the previous close of 98.17.

The offshore yuan was trading at 7.0978 per dollar as of midday.

(Reporting by Winni Zhou and John Ruwitch; Editing by Simon Cameron-Moore & Kim Coghill)