By Nicole Friedman

NEW YORK--U.S. oil futures declined Tuesday as concerns about falling gasoline demand weighed on prices, while ongoing geopolitical tension kept the global benchmark trading near flat.

Light, sweet crude for September delivery fell $1.07, or 1.1%, to $100.60 a barrel on the New York Mercantile Exchange. Brent crude on ICE Futures Europe recently traded down 10 cents, or 0.1%, at $107.47 a barrel.

U.S. prices slid Tuesday on concerns that demand for petroleum products, particularly gasoline and other fuels, will fall as Americans take fewer vacations once the summer ends. The busy summer-driving season, when gasoline demand is typically strongest, ends on Labor Day.

Oil supplies have fallen in recent weeks as refineries have run at unusually high utilization rates. But gasoline supplies have increased in the past three weeks.

The U.S. Energy Information Administration is due to report its storage data for the week ended July 25 on Wednesday at 10:30 a.m. ET. While analysts and traders are still finalizing their forecasts, many expect the agency to report that gasoline stockpiles grew for another week.

"It's pretty clear that if we keep our utilization rates pretty high, we do drop our crude stocks down, but you just see them go into the storage tanks for products," said Gene McGillian, broker and analyst at Tradition Energy in Stamford, Conn.

Meanwhile, global benchmark Brent is trading near flat. Globally, oil is well supplied and demand from refiners in Europe and Asia is weak. But ongoing violence in hotspots around the world is keeping traders concerned that a supply outage could quickly shrink global inventories.

The closely watched price gap between the U.S. benchmark contract and Brent recently traded at $6.87 a barrel, the highest level since early July.

President Barack Obama and European Union leaders agreed Monday to adopt new sanctions against critical sectors of Russia's economy. Russia is the second-largest oil exporter, and worries that those exports could fall have buoyed Brent prices in recent months.

However, "the impact on energy should be minimal in the short term," said Citigroup in a note, referring to the latest sanctions. Restrictions in technology exports to Russia could limit the country's longer-term production capacity, the bank said.

"The costs to Moscow come in the form of the threats that sanctions imply for long-term oil and gas production in Russia, upon which the government depends for revenues," according to Citigroup.

Front-month August reformulated gasoline blendstock, or RBOB, recently rose 2.31 cents, or 0.8%, to $2.8723 a gallon. August diesel rose 1.93 cents, or 0.7%, to $2.9072 a gallon.

Write to Nicole Friedman at nicole.friedman@wsj.com

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