WASHINGTON—The number of Americans filing applications for new unemployment benefits fell last week to the lowest level in nearly 45 years, a sign the labor market is beginning 2018 with strong momentum.

Initial jobless claims, a proxy for layoffs across the U.S., fell last week to a seasonally adjusted 220,000 in the week ended Jan. 13, the Labor Department said Thursday. This marked the lowest level for claims since February 1973.

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