(Adds details, comments and background) SHANGHAI, Aug 19 (Reuters) - The Chinese yuan eased to a three-week low against a firmer dollar on Thursday, while its value against the currencies of major trading partners touched a fresh 5-1/2-year high. The People’s Bank of China (PBOC) set the midpoint at 6.4853 yuan per dollar prior to the market open, 62 pips or 0.1%, stronger than the previous fix of 6.4915. The stronger official guidance rate has pushed China’s trade-weighted yuan basket index to 98.88, the highest since March 10, 2016, and up 4.26% so far this year, according to Reuters’ calculations based on official data. Analysts and traders said the firmer basket index reflected the yuan’s resilience against the dollar relative to its peers. The dollar index gained about 1.4% this month amid talk of the U.S. Federal Reserve’s plan to taper its asset purchases, piling downward pressure on emerging market currencies, but the yuan only weakened 0.44% to the greenback. The spot market opened at 6.4845 per dollar and was changing hands at 6.4932 as of 0219 GMT, 91 pips weaker than the previous late session close. Despite the slight weakness in the spot yuan, Ken Cheung, chief Asian FX strategist at Mizuho Bank, said the yuan’s broad strength against the basket showed that the central bank still wanted a stable yuan for the time being.A breach of the psychologically important 6.5 per dollar would be a clear signal the People’s Bank of China (PBOC) had started to allow more weakness in the local currency, he added. “Markets will closely watch the PBOC’s actions after the annual Jackson Hole policy symposium,” Cheung said. Many market analysts and traders expect the Fed to reveal the timing of tapering at the annual conference of central bankers next week. (Reporting by Winni Zhou and Andrew Galbraith; Editing by Shri Navaratnam and Ana Nicolaci da Costa)