By Georgi Kantchev And Timothy Puko 

Oil prices slid to fresh multi-month lows on Monday, with bearish supply-and-demand data contributing to the market's gloomy mood.

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. Meanwhile, the number of oil drilling rigs in the U.S. rose last week, fueling concerns that the supply glut is unlikely to abate soon.

"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.

Light, sweet crude for September delivery recently fell 1.9%, $46.21 a barrel on the New York Mercantile Exchange. It is within $3 of the six-year low settlement price it hit in March.

Brent, the global benchmark, fell even more, 3.4%, to $50.46 a barrel on ICE Futures Europe. It is within $4 of the six-year low settlement price it hit in January.

In China, 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.

"The main oil demand growth engine of the world ... China may not be leading the global oil rebalancing effort anytime soon," Dominick Chirichella, analyst at the Energy Management Institute, said in a note. "Supply is likely to remain robust for the foreseeable future prolonging the long awaited rebalancing of the global oil market."

Oil entered a bear market last month with the U.S. benchmark shedding 21% of its value and Brent losing 18%. Persistently high U.S. output and record production from other major suppliers, combined with a repeated effort from U.S. producers to get more drill rigs back to work, has dashed optimism that the market could rebalance soon.

U.S. government data on Friday suggested that the country's oil production peaked in March, but the U.S. oil rig count--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.

The data suggests that "U.S. producers are coming to the point where they start considering growth, at least at the margin" said analysts at Deutsche Bank.

Looking beyond the U.S., many suppliers around the globe also seem likely to expand production.

Barclays said a group of 101 oil companies that it tracks, which cover around 40% of global oil production, show 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 recently fell 3.4% to $1.7112 a gallon. Diesel futures fell 1.9% to $1.5588 a gallon.

Biman Mukherji contributed to this article.

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

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