Oil prices fell by more than 1 percent on Monday as U.S. drilling activity picked up, even as a refinery fire in the U.S. state of Illinois resulted in the shutdown of a large crude distillation unit.
U.S. West Texas Intermediate (WTI) crude futures were at $51.92 per barrel at 0144 GMT, down 80 cents, or 1.5 percent, from their last settlement.
International Brent crude oil futures were down 71 cents, or 1.1 percent, at $61.39 a barrel. In the United States, energy firms last week increased the number of oil rigs operating for the second time in three weeks, a weekly report by Baker Hughes said on Friday.
Companies added 7 oil rigs in the week to Feb. 8, bringing the total count to 854, pointing to a further rise in U.S. crude production, which already stands at a record 11.9 million bpd.
WTI prices were also weighed down by the closure of a 120,000 barrels per day (bpd) crude distillation unit (CDU) at Phillips 66s Wood River, Illinois, refinery following a fire on Sunday.
Elsewhere, the head of Russian oil giant Rosneft, Igor Sechin, has written to the Russian President Vladimir Putin saying Moscows deal with the Organization of the Petroleum Exporting Countries (OPEC) to withhold output is a strategic threat and plays into the hands of the United States.
The so-called OPEC+ deal has been in place since 2017, aimed at reining in a global supply overhang. It has been extended several times and, under the latest deal, participants are cutting output by 1.2 million bpd until the end of June.
OPEC and its allies will meet on April 17-18 in Vienna to review the pact.
Preventing crude prices from falling much further have been U.S. sanctions on Venezuela, targetting its state-owned oil firm PDVSA.