Oil futures were trading lower Monday as an increase in the number of rigs actively drilling for oil in the U.S. and rising crude output from major producers pushed prices in New York back below the $50-a-barrel level.

November West Texas Intermediate crude CLX26, fell by 54 cents, or 1.1%, to $49.81 a barrel on the New York Mercantile Exchange. A settlement around this level would be the lowest in just over a week. December Brent crude UK:LCOZ6 on London’s ICE Futures exchange lost 50 cents, or 1%, to $51.45 a barrel.

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“The U.S. [oil] rig count has steadily risen over the past weeks, to totals not seen since February,” said Daniel Holder, commodity analyst at Schneider Electric, adding that since June, oil producers have added over 100 rigs to last week’s count of 432.

On Friday, data from Baker Hughes US:BHI showed that the number of active U.S. rigs drilling for oil climbed by 4 for the week.

“A rise in U.S. drilling, combined with a strong dollar and record OPEC output,” sent prices lower Monday,” said Phil Foster, managing director of U.K.-based energy consultancy Love Energy. “Concerns over slowing in the global economy eroding demand for fuel also depressed the market.”

The ICE U.S. Dollar Index DXY, -0.02% rose about 1.5% last week, pressuring prices for dollar-denominated oil prices.

A report from the Organization of the Petroleum Exporting Countries issued last week, meanwhile, showed that production from its members rose to 33.39 million barrels a day in September, while the International Energy Agency pegged output at 33.64 million barrels for that month.

OPEC member Iran has been “steadily increasing production since the embargo was lifted against the nation, and its 4 million barrel per day production target could actually be hit by the end of the month,” said Holder. “Because this target production will be achieved by the November OPEC meeting, Iran will be in a comfortable position to accept a[n output] freeze.”

The plan the group announced in late September, which won’t be completed until the end of November, targets output of no more than 33 million barrels a day.

Following the announcement of the plan, data from the U.S. Commodity Futures Trading Commission’s Commitments of Traders report showed that as of Oct. 11, money managers were buyers of 38,690 contracts of WTI crude oil, according to Tim Evans, energy analyst at Citi Futures. The net long position of 292,083 contracts was the most since July 2014. Long positions are essentially bets that prices will rise.

Still, BMI Research notes a major hurdle to the OPEC plan would be deciding whose estimates to use when it comes to determining individual production quotas.

“Negotiating individual country quotas will likely reignite tensions in the group and the growing discrepancies between official and third party production estimates are a cause for concern,” the firm added.

Ahead this week, the market will watch for the weekly U.S. crude inventories and production data to be released on Wednesday. China will also release its monthly crude production and throughput data on the same day. On Friday, the Chinese customs is expected to release a detailed breakdown of China’s September crude imports and oil exports. Preliminary data showed China’s crude imports rose 18% on-year to 33.06 million tons, or 8.08 million barrels a day in September, while exports of oil products rose 21% on-year to 4.3 million tons.

Back on Nymex, November gasoline US:RBX6 tacked on just under a penny to $1.501 a gallon while November heating oil US:HOX6 shed 1.4 cents, or 0.9%, to $1.554 a gallon.

November natural gas US:NGX16 traded at $3.244 per million British thermal units, down 4.1 cents, or 1.3%.

— George Stahl contributed to this article.