Opec is pumping at more than 95 percent of capacity, the highest for a quarter of a century, giving it little room for maneuver in an emergency.

US light crude struck $43.34 a barrel, the highest in its 21-year history on the New York Mercantile Exchange. It was last trading at $43.18, up 43 cents.

In London, Brent crude scaled heights not seen since the Gulf War, peaking at $39.90, before trading at $39.68.

Oil staged its first assault on historic highs Wednesday after news Russian oil giant Yukos might face a ban on oil sales while courts try to enforce a multi-billion-dollar tax debt.

Prices retreated after Thursday's reprieve by the justice ministry allowed Yukos to keep pumping, but the market remains on edge over the company's fate.

"There's a sense with Yukos of postponing the inevitable. Yukos's financial problems will get worse in the coming weeks and the market is very nervous that we will see some of its 1.7 million barrels a day shut in for some period," said Steve Turner, an oil analyst at Commerzbank Securities.

Yukos has said the company could collapse by mid-August because of a freeze on its bank accounts and assets, adding that its rail shipments of oil, which make up a quarter of its total sales, could be affected soon.

Yukos, whose former CEO Mikhail Khodorkovsky is on trial for tax evasion and fraud, pumps a fifth of Russian oil.

IRAQ VIOLENCE FEAR

Traders also remain wary over accelerating Iraqi oil flows after repeated export disruptions this summer.

"Problems could always occur in Iraq. It's difficult to see someone turning a switch and the situation changing there," Turner added.

The Iraqi oil minister said Thursday the country's oil exports would average between 1.7 million and 1.8 million barrels per day (bpd) next month, from 1.5 million in July.

However, Baghdad has consistently missed higher targets due to a spate of pipeline sabotage attacks in the south.

Even short-term outages this summer in major producers Norway and Nigeria have bolstered price strength, as traders fear Opec could be unable to compensate for even minor hiccups as oil demand grows at its fastest in more than two decades.

The head of the International Monetary Fund said Thursday that the spurt in demand showed no signs of impeding the global economic recovery.

Rodrigo Rato said the recovery had gathered enough momentum to weather the effects of rising oil prices, which he said was caused by both economic and political factors.

Rato said he agreed with the Paris-based International Energy Agency that oil prices would stay higher than was expected "only a few months ago."

The IMF and economists have forecast global growth in excess of 4.5 percent this year even as oil prices rise and more increases in US interest rates loom.