Lower U.S. equities following the U.S. Supreme Court decision upholding key elements of President Barack Obama's healthcare reform law added to the day's pressures on crude futures.

Despite recent gains, Brent crude futures were on track to end the second quarter down about 25 percent, the largest drop since the last quarter of 2008, which was the height of a financial crisis.

U.S. crude futures, meanwhile, were headed for drop of fall about 24 percent, also the biggest quarterly loss since the last quarter of 2008.

Trading on Wall Street was volatile, with large health insurers hit, after the Supreme Court upheld the centerpiece "individual mandate" provision of the Obama healthcare overhaul. Equities were already lower on skepticism that the European Union Summit will result in concrete measures.

U.S. equities later ended with pared losses on talk that EU finance officials were working on urgent measures to ease financial pressure on Spain and Italy.

"It's the euro zone problems, the strength of the dollar and the weak equities," said Stephen Schork, president of the Schork Group in Villanova, Pennsylvania, commenting on the reasons for the day's price drop.

The euro fell against the dollar on doubts about the outcome of the EU summit, denting investors' interest in buying riskier assets such as oil and other commodities. <USD/>

In London, Brent crude oil futures for August settled $2.14 lower at $91.36 a barrel, after a session low of $90.88.

U.S. August crude fell to an eight-month low, closing at $77.69 a barrel, down $2.52.

"The further fall in equities triggered by the Supreme Court decision ... exerted selling pressure on crude futures," said Addison Armstrong, director of market research at Tradition Energy in Stamford, Connecticut.

Technical support for U.S. crude at around $79 was breached by selling after equities slid further, Armstrong said.

Trading volumes were light, with Brent crude nudging up 1 percent against its 30-day average while U.S. crude volume was down 2.5 percent against its 30-day average, according to Reuters data.

Brent's premium against U.S. crude widened to $13.67 against $13.29 on Wednesday, with U.S. crude having fallen faster than its London counterpart.

Economic data released on Thursday also weighed on crude futures. U.S. GDP for the first quarter grew at a 1.9 percent annual pace, while consumer spending and export growth were not as robust as previously thought.

Another set of data showed that weekly jobless benefit claims fell, but still remained too high, reflecting weakness in the job market.

And in Europe, euro zone economic sentiment soured in June as managers of businesses and in factories across the currency area saw little reason for cheer as the region's economy stalls, even in its wealthier, northern nations.

In Wednesday's trading, crude futures rose on a strike by Norwegian oil workers that reduced North Sea oil output and data that showed U.S. crude and distillate stockpiles declined last week. In overnight Asian trading, crude futures initially rose on Thursday on those factors, but later retreated amid negative perception about the EU summit.

LOW EXPECTATIONS

EU leaders began a meeting in Brussels on Thursday openly divided, with German Chancellor Angela Merkel pitting herself against France and Italy and insisting they put the bloc's fundamental problems ahead of emergency action.

But French President Francois Hollande said he expected agreement on emergency steps to help euro zone partners whose borrowing costs have reached unsustainable levels.

"Market sentiment is very negative," said Carsten Fritsch, a commodities analyst at Commerzbank in Frankfurt. "There are concerns that the EU summit will disappoint."

Analysts say the euro zone debt and financial crisis is stifling activity in the region, eroding investor confidence and dampening economic growth outlook in other parts of the world.

NORWAY STRIKE

Norwegian oil production has been further cut by 290,000 barrels per day, according to a union official, from 240,000 bpd earlier this week, as an oil workers' strike that began on Sunday continued, with no signs of a resolution.

But the government of Norway, the world's eighth-largest oil exporter, has said it is not about to intervene, a local industry association said.

State-controlled oil and gas company Statoil (STL.OL) shut its key Oseberg field center on Sunday and other fields including Statoil's Heidrun field and BP's (BP.L) have also been affected by the strike.

IRAN SANCTIONS

Crude futures have fallen about 30 percent from the year's high of $128.40 for Brent and $110.55 for U.S. crude, both occurring in March, despite an embargo of Iranian crude beginning July 1.

The fall in prices has been largely due to a rise in the output from the Organization of Petroleum Exporting Countries, mostly from Saudi Arabia, which is countering the threat of a sudden drop in Iranian supplies.

In a meeting earlier this month, however, OPEC agreed to adhere to its output target of 30 million bpd set in December.

But Venezuela's energy minister said that OPEC could hold an extraordinary meeting in the third quarter of this year if global crude prices remain low.

Venezuela wants the group to set an oil price band of $80 to $120 a barrel to stem crude's recent tumble, seeking to revive a policy the cartel scrapped seven years ago.

Iran is at loggerheads with world powers over its disputed nuclear program that earlier this year had raised fears of that Tehran would block the Strait of Hormuz, a vital oil shipping lane.

Keeping an eye on Iran, Saudi Arabia has reopened an old oil pipeline built by Iraq to bypass Gulf shipping lanes, giving Riyadh the means to export more of its crude from Red Sea terminals should Iran try to block the strait, industry sources said.

Meanwhile, the Obama administration, which has imposed global financial sanctions intended to curtail business with Iran, has granted exceptions to the sanctions to China and Singapore, Secretary of State Hillary Clinton announced.