The number of Americans filing for unemployment benefits rebounded from a 45-year low last week, though by less than expected, pointing to tightening labor market conditions.

Initial claims for state unemployment benefits increased 17,000 to a seasonally adjusted 233,000 for the week ended Jan. 20, the Labor Department said on Thursday. Claims fell to 216,000 in the prior week, the lowest level since January 1973.

Economists polled by Reuters had forecast claims rising to 240,000 in the latest week. Claims have been volatile recently because of the difficulty adjusting the data for seasonal fluctuations at the end of 2017 and the start of the new year. Unseasonably cold temperatures also had an impact on the data.

The Labor Department said claims for Maine were estimated. It also said claims-taking procedures in Puerto Rico and the Virgin Islands had still not returned to normal months after the territories were pummeled by Hurricanes Irma and Maria.

Last week marked the 151st straight week that claims remained below the 300,000 threshold, which is associated with a strong labor market. That is the longest such stretch since 1970, when the labor market was much smaller.

"The song remains the same for tightness of the labor market - employers are extremely reluctant to fire current workers, which reflects not only the current positive business environment but also the difficulty in finding qualified replacements," said John Ryding, chief economist at RDQ Economics in New York.

The U.S. dollar was largely unchanged against a basket of currencies after the data. Prices of U.S. Treasuries were trading mostly weaker, while U.S. stock index futures were higher.