WASHINGTON - The number of U.S. workers filing new claims for jobless benefits unexpectedly fell 21,000 last week, government data showed on Thursday, and a less volatile unemployment claims gauge dipped to an eight-month low.

Initial claims for state unemployment insurance declined to a seasonally adjusted 530,000 in the week ending Sept. 19 from a revised 551,000 in the previous week. Analysts polled by Reuters were expecting claims to rise to 550,000 from a previously reported 545,000.

The four-week moving average of new claims dipped to 553,500, the lowest since 547,000 in the week ending January 24.

The week's decline was affected by seasonal factors, a Labor Department analyst said. Seasonal adjustments anticipate a rise in claims in the week after the Labor Day federal holiday, and an unadjusted increase in claims fell short of the expected increase, the official said.

Continued claims of workers still on jobless aid after an initial week of benefits fell by a bigger-than-expected 123,000 to 6.138 million in the week ending Sept. 12. Analysts were expecting claims to dip to 6.19 million. (Reporting by Mark Felsenthal, Editing by Andrea Ricci)