WASHINGTON, Sept 21(Reuters) - U.S. homebuilding increased more than expected in August, likely boosted by an easing in lumber prices, though land and labor shortages remain challenges. Housing starts advanced 3.9% to a seasonally adjusted annual rate of 1.615 million units last month, the Commerce Department said on Tuesday. Data for July was revised up to a rate of 1.554 million units from the previously reported 1.534 million units. Economists polled by Reuters had forecast starts rebounding to a rate of 1.555 million units. (Reporting By Lucia Mutikani; Editing by Andrew Heavens)