By Kate Davidson and Sarah Portlock 

WASHINGTON--The number of Americans filing new claims for jobless benefits plunged last week to a 15-year low, though the reading may have been skewed by seasonal volatility.

Initial jobless claims decreased by 43,000 to a seasonally adjusted 265,000 in the week ended Jan. 24, the Labor Department said Thursday. That was the lowest level since April 2000. Economists surveyed by The Wall Street Journal had expected 300,000 new claims.

The drop could be because of the shortened workweek from the Martin Luther King Jr. holiday, a Labor Department analyst said. There was nothing unusual in the state data to suggest that the numbers were an anomaly, the analyst added.

The weekly claims readings can be noisy from Veterans Day through Presidents Day, Amherst Pierpont Securities Chief Economist Stephen Stanley said in a note to clients.

"Swings like this one should always be taken with a sizable grain of salt," he said. "In about a month's time, the weekly readings should begin to settle down."

Readings around the holiday period can show a spike in layoffs as companies hire, then soon dismiss, temporary workers. The claims data is seasonally adjusted to account for the variations, but the week-to-week data can still be volatile at other times.

The prior week's figure was revised up to 308,0000 claims from an initially reported 307,000.

The four-week moving average for initial claims, which evens out weekly volatility, fell by 8,250 to 298,500. Thursday's report showed the number of people filing continuing claims for unemployment benefits decreased by 71,000 to 2.4 million for the week ended Jan. 17.

Initial jobless claims are still near historic lows and are consistent with an economy that is adding jobs.

Some economists have been cautioning against extrapolating from the recent increase in claims, but the rise over the past few weeks did cause some concern that momentum in the job market had slowed.

"The drop in the latest week is a relief--even if it was exaggerated a little by seasonal adjustment problems," Jim O'Sullivan, chief U.S. economist at High Frequency Economics, said in a note to clients.

Initial claims slipped below 300,000 last summer and mostly remained below that level for the rest of 2014. The last time claims had a similar streak was in 2000. A separate Labor report showed 2014 was the best year for hiring since 1999

Other signs show the labor market improving. The unemployment rate fell to 5.6% in December, its lowest level since 2008, and employers have added more than 200,000 jobs for 11 straight months.

The Federal Reserve on Wednesday acknowledged the labor market's progress, which the central bank is watching closely as it weighs when to lift short-term interest rates.

"Labor market conditions have improved further, with strong job gains and a lower unemployment rate," the Fed said in a statement after its policy meeting.

Write to Kate Davidson at kate.davidson@wsj.com and Sarah Portlock at sarah.portlock@wsj.com