Global oil markets extended declines again Monday, taking crude prices to the lowest levels since July of last year, even as OPEC leaders vowed over the weekend to extend their planned production cuts if they fail to re-balance global crude stocks.

Brent crude oil, the global benchmark, fell more than 10% last week to trade at the lowest levels since the summer of 2017 amid concern that a slowing world economy, coupled with record output from the United States, Russia and Saudi Arabia, would continue to offset the 1.2 million barrel per day output cut agreed by OPEC members earlier this month in Vienna. Against that backdrop, which has seen crude fall more than 38% since its early October peak, the United Arab Emirates energy minister told an industry conference Sunday that OPEC could re-visit its production cut agreement if it doesn't have an impact of prices. 

"If we are required to extend for (another) six months, we will do it, I can assure you an extension will not be a problem," Suhail al-Mazrouei said, adding the place is "well studied", but insisted that if it does not work, "we always have the power in OPEC to call for an extraordinary meeting." 

Brent crude contracts for February delivery, the global benchmark, were marked 63 cents lower from their Friday close in New York and changing hands at $53.19 per barrel in early European trading, marking a decline of more than 38% from the October 3 peak of $86.74 per barrel.

WTI contracts for the same month, which are more tightly liked to U.S gas prices, were seen $1.04 lower at $44.54 per barrel, around 40% down from its early October peak.

Oil majors edged higher in European trading, however, with BP plc (BP) - Get Report   closing 0.75% higher while domestically-listed rival Royal Dutch Shell plc (RDS.A) was little-changed at £25.165 each. France''s Total SA (TOT) - Get Report slipped 1.12% to €45.53 and has fallen more than 18% over the past three months.

Despite the ongoing fall in crude prices, U.S. drillers added 10 new installations last week -- the first additions in nearly a month -- to take the total rig count to 883, according to the benchmark survey from Houston-based oil services provider Baker Hughes, up from 747 during the same time last year.

Baker Hughes's count showed that more than half of those units are operating in the Permian Basin, suggesting falling crude prices aren't keeping drillers from tapping into the biggest shale oil area in the United States. 

The U.S. Energy Information Administration said earlier this month that production from the country's seven biggest shale areas, including the Permian Basin, is likely to top a record 8 million barrels per day this year before rising to around 8.1 million in early 2019. That's likely to add to overall U.S. output, which hit a record 11.7 million barrels per day in the week ending December 7.