(Reuters) -Starbucks ( SBUX ) on Tuesday reported a surprise drop in second-quarter comparable sales, as it battled weak demand for its coffees in the United States and China, its two biggest markets, sending its shares down 6% in extended trading.

Demand in the United States decelerated for the second straight quarter as boycott movements impacted traffic at its stores, while a choppy macro environment weighed on demand for its pricier beverages.

While the company works on solidifying its position in the premium coffee market in China, it still grapples with increased competition from mass market brands, which offer coffee that's easier on the pocket for cautious consumers.

The coffee chain's second-quarter global comparable sales fell 4%, compared with a 1.44% rise expected on average by analysts, according to LSEG data.

(Reporting by Juveria Tabassum; Editing by Anil D'Silva)