Oil prices extended losses on Monday, starting off the last week of January with a red tally, as increasing oil rigs in the United States fueled concerns over a potential supply glut.

Oil rigs in the U.S. oilfields rose for the first time since December 2018 to 862 last week, up 10 rigs from the previous week ending January 18, Houston-based energy services firm Baker Hughes said on Friday.

Last week's rally in oil rigs also marked an increase of over 100 rigs compared with the same period last year, further deepening fears of excessive oil output in the future.

To calm rattled investors, Saudi energy minister Khalid al-Falih told CNBC on Monday that Russia, one of the world's top oil producers, has promised to "pick up the pace" in production cuts this year.

The Saudi-led, 14-member OPEC and 10 allies reached a deal in December to slash a combined 1.2 million barrels per day (bpd) for the first six months of 2019, in an effort to contain the constantly rising global outputs.

Moscow previously said it would cut output by 50,000 to 60,000 bpd in January, which is still a long distance from its goal of slashing 230,000 bpd in the first six months of 2019, according to the latest data released by OPEC on Friday.

The West Texas Intermediate for March delivery fell by 1.7 U.S. dollars to settle at 51.99 U.S. dollars a barrel on the New York Mercantile Exchange, while Brent crude for February March fell by 1.71 U.S. dollars to close at 59.93 U.S. dollars a barrel on the London ICE Futures Exchange.