By Timothy Puko And Georgi Kantchev 
 

The world's benchmark oil price fell to less than $50 a barrel Monday for the first time in six months as a sluggish global economy and rampant oil boom keep crude markets falling.

Monday's selloff widens losses from July that plunged oil into a bear market. Record production in the U.S. has led to an international competition to produce even more, cut prices and fight for customers around the world. The past week brought signs that production is still going strong, dashing hopes that low prices may force producers to slow down.

Monday brought ominous signs on the demand side, suggesting the global economy may not ramp up enough to absorb all the oil. Chinese manufacturing activity fell to a two-year low, according to data released Monday, clouding the demand outlook for the world's second-biggest oil consumer. Brazil, India, and Russia also slowed, further indications of tepid demand growth around the world, wrote Tim Evans, analyst at Citi Futures Perspective in New York, in a note.

"The prospects of a second half-year price rebound have evaporated and there is a clear and present danger of prices revisiting the previous lows of the year," said David Hufton of oil brokerage PVM.

Brent, the global benchmark, took the larger losses of the day, falling, $2.69, or 5.2%, to $49.52 a barrel on ICE Futures Europe. It was its largest one-day losses since July 6 and the lowest settlement since Jan. 29.

Light, sweet crude for September delivery settled down $1.95, or 4.1%, to $45.17 a barrel on the New York Mercantile Exchange. It is within $2 of the six-year low settlement price it hit in March.

U.S. crude has now halved its deficit to Brent from this year's peak, down to less than $5 a barrel. It could portend a big shift to come. The U.S. economy has been stronger than the global economy, and U.S. production has shown signs of peaking while international rivals are still pushing their record pace.

Bank of America Corp. strategists said late Friday that the U.S. benchmark could surpass Brent by next year. U.S. refiners, which are geared to make gasoline, will need to import crude to keep running at high rates, so domestic prices will rise to attract foreign barrels, Bank of America Merrill Lynch forecast.

"Ground zero from the global surplus may be moving away from the United States and North America to the global community," Dominick Chirichella, analyst at the Energy Management Institute said Monday.

He said that data on China and the world economy started Monday's oil rout. The Caixin manufacturing purchasing managers index, a gauge of nationwide manufacturing activity, fell to 47.8 in July from 49.4 in June, the lowest level of the index since 2013. A level under 50 indicates a contraction in activity.

Mr. Chirichella said global prices took a further hit after proclamations from Iran that it could double its production once its new nuclear pact allows it to export oil again. It adds to ongoing fears that the Organization of the Petroleum Exporting Countries is ramping up production to record levels.

Those factors have been major players as oil slipped into a bear market last month. U.S. oil is now down 24% since June 30 and Brent is down nearly 22% since then.

U.S. government data Friday suggested that the country's oil production peaked in March, but the U.S. oil rig count, which is a rough proxy for activity in the industry, rose last week for the third time in the last four weeks, according to Baker Hughes Inc. The count rose by five rigs to 664, on top of a 20-rig increase in the previous week.

That added more pressure again Monday. Persistently high U.S. output and record production from other major suppliers, combined with a repeated effort from U.S. shale-drillers to get more rigs back to work has dashed optimism that the market could rebalance soon.

"The market is very skeptical of shale production declines," said Sabine Schels, a commodities strategist with Bank of America Merrill Lynch in London. "We are probably a few dollars away" from a bottom. Many suppliers around the globe also seem likely to expand production.

Barclays said a group of 101 oil companies that it tracks, which covers around 40% of global oil production, shows no slowdown in the pace of production growth in 2015. After growing by one million barrels a day in 2014, the companies plan to accelerate output growth to 1.4 million barrels a day this year and maintain that level into 2016.

Gasoline futures settled down 9.75 cents, or 5.5%, at $1.6745 a gallon, its lowest settlement since Feb. 24. Diesel futures fell 5.84 cents, or 3.7%, to $1.5305 a gallon, its lowest settlement since July 2009.

--Biman Mukherji and Nicole Friedman contributed to this article.

Write to Timothy Puko at tim.puko@wsj.com and to Georgi Kantchev at georgi.kantchev@wsj.com

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