Energy firms pulled another 13 rigs from U.S. oil fields this week, the biggest drop in four weeks, data showed on Friday, showing that a near six-month slump in activity had yet to run its course despite a rebound in crude oil prices.

That was the 25th straight weekly decline, bringing the total rig count down to 646, the lowest since August 2010, oil services company Baker Hughes Inc said in its closely followed report.