Oil futures on Friday were on course for their biggest weekly percentage declines since January of just under 10 percent as signs of tensions resurfaced between Saudi Arabia and Iran that could scupper a key supply cut pact.
Traders also noted a surge in U.S. crude inventories last week and muted demand continued to weigh on futures.
U.S. West Texas Intermediate (WTI) futures were down 59 cents, or 1.3 percent, at $44.07 a barrel, having fallen as low as $43.57 earlier in the session. For the week, it was down more than 9 percent, the biggest decline since mid-January.
Brent crude futures were down 74 cents, or 1.6 percent, at $45.61 per barrel at 2:28 p.m. ET (1838 GMT), off a low of $45.08. It was on pace to end the weed down more than 8 percent.
At a meeting of OPEC experts last week, Riyadh threatened to raise oil output steeply to bring prices down if Tehran refused to limit its production, a source from the Organization of the Petroleum Exporting Countries (OPEC) told Reuters.
A Gulf OPEC source, however, said Saudi Arabia "did not threaten" anyone with production increases at an OPEC experts meeting last week, but the source warned that production around the world will rise if there is no output limiting deal.
OPEC Secretary General Mohammed Barkindo denied that the Saudis threatened to raise output, Bloomberg News reported. Prices sharply pared losses on the report, but resumed their slide shortly after.
The OPEC meeting was intended to work out the details of cuts ahead of the next OPEC meeting on Nov. 30 following a decision to reduce output in Algiers to 32.5 to 33 million barrels per day in order to boost prices.
"We remain consistent in our long standing view that OPEC will be unable to patch together a deal capable of placing even a minor dent in global oversupply," Jim Ritterbusch, president of Chicago-based energy advisory Ritterbusch & Associates, said in a note.
The U.S. oil rig count resumed its rise following its first dip in roughly four months in the previous week. U.S. drillers added 9 rigs in the last week for a total of 450. At this time last year, there were 572 rigs in operation.
Analysts said markets were also weighed down by traders pulling out money from futures ahead of the U.S. presidential election on Tuesday, which is seen as a risk to markets.
Beyond election concerns, traders said fundamentals were weak, with U.S. crude stocks surging, demand growth low, and doubts that OPEC and Russia can agree on a meaningful output cut this month.
North Sea crude exports are also set to rise significantly in December, adding to a surplus of light, sweet grades in the market. The sudden outage of some 200,000 barrels per day of alternative light Nigerian crude on Wednesday garnered only mild attention.
While oil production remains near records and inventories are high, British bank Barclays said demand growth was timid.
Demand growth over July-September was less than a third that of the year-ago quarter, Barclays said in a note, estimating last quarter's growth below 1 million barrels per day (bpd).
The consumption rise for the last quarter will not be much higher, before averaging 1.3 million bpd in 2017, it added.
In the United States, crude oil stockpiles soared more than 14 million barrels last week, the largest build on record, highlighting that a global fuel supply overhang is far from over.
Also in the United States, the Colonial Pipeline carrying gasoline, which was disrupted this week by an explosion, is expected to restart Line 1 on Sunday afternoon.
— CNBC's Tom DiChristopher contributed to this report.