Brent crude oil prices fell by $1/bbl to $72.75 on Sept. 4 because of pessimism about demand in the coming months as crude producers offered mixed signals about supply increases.

Brent crude futures were down 88 cents, or 1.2%, to $72.87 at 9:40 a.m. CDT. U.S. West Texas Intermediate crude futures were down 90 cents, or 1.28%, at $69.44.

Both benchmarks had earlier lost $1 and then bounced back to gain $1 from Sept. 3's closes, following news OPEC+ was discussing delaying a possible output increase because Libyan production is expected to rise.

A broader sell-off had seen prices for Brent crude futures tumble as much as 11%, or about $9, in a little over a week, hitting a low of $72.63 on Sept. 4.

Lackluster data from the U.S. and China had strengthened persistent expectations of a weaker global economy and oil demand, helping set off a broader decline in world markets.

"It's definitely worries about a slowdown in manufacturing," said Phil Flynn, senior analyst at Price Futures Group. "That's the only negative we're seeing."

Meanwhile, traders believed there could be an end in sight to a dispute halting Libyan oil exports, bringing more crude supply back online.

That posed a challenge for OPEC+, which last week looked set to proceed with planned output hikes in October. The production group is now concerned about the market volatility, one of the sources said, and a delay to the hikes is being discussed.

"If OPEC+ does not provide reassurance that current output cuts would be extended more indefinitely, then the market could lose faith in OPEC+ defending the $70/bbl level," Citi analysts wrote in a note.

Recent data releases fueled wider concerns around weaker than expected demand from China, the world's biggest crude importer, and U.S. consumption taking a hit.

Chinese data on Aug. 31 showed manufacturing activity sank to a six-month low in August, when growth in new home prices slowed.

Meanwhile in the U.S., Institute for Supply Management data on Sept. 3 showed manufacturing remained subdued.

Weekly U.S. inventory data has been delayed by Sept. 1's Labor Day holiday. The report from the American Petroleum Institute is due at 4:30 p.m. EDT (2030 GMT) on Sept. 4 and data from the U.S. Energy Information Administration will be published at 11:00 a.m. EDT (1500 GMT) on Sept. 5.

U.S. crude oil and gasoline stockpiles were expected to have fallen last week, a preliminary Reuters poll showed. 

While traders were pessimistic on demand fears, changes in supply could easily change sentiments, Flynn said.

"We could flip on a dime," he said. "It could very easily turn positive. We could see a pretty decent crude draw later today."