The price of crude oil has retreated slightly after hitting record highs above $142 a barrel, amid concerns that supply will not meet demand.
In London, Brent crude was trading at $140.16, having earlier hit $142.13.
New York light crude had climbed as high as $142.26 a barrel, but later fell back to $140.34.
Producers' group Opec has been under pressure to boost production, though recent reports have shown its members are split over whether to lift output.
Libya has threatened to cut production because the market is well supplied.
Libya's most senior oil official, Shokri Ghanem, said on Thursday he was looking into the possibility of cutting production in response to US threats against oil producers.
Analysts blame the price of crude on a variety of factors from basic supply and demand to hedge funds.
Opec has said speculators have played a part in the oil spike this year, but others are not convinced.
"We believe the factors driving oil prices higher are fundamental and not speculative," Deutsche Bank said in a research note.
"Oil needs to rise to $150 a barrel for oil as a share of global Goss Domestic Produce to reach the levels that occurred in the early 1980s," according to the bank.
But tensions between oil consumers and producers are rising.
The US House of Representatives has passed a bill that would allow the Justice Department to sue Opec members for limiting supplies.
But the bill has yet to be backed by the Senate and the White House has already said it would veto the bill.
There was also scepticism about whether there will actually be a cut in Libya, because of soaring prices.
"I doubt that any real effort in cutting output would be forthcoming, considering that pricing continues to hit new records," said Victor Shum, an analyst at Purvin & Getz.
'Radically new level'
Meanwhile, the chief executive of Gazprom, Alexei Miller, has been talking down the influence of Opec.
Saying that Opec had no real impact on prices, he told the Financial Times: "Not a single decision has been passed of late that would really influence the global oil market."
He also said that the world was undergoing "a great surge in oil and gas prices, which will end with prices at a radically new level".
Mr Miller predicted that Gazprom would become the most influential company in the energy business.
On Friday, the firm approved the replacement of former chairman Dmitry Medvedev, who is now Russian president, with former prime minister Viktor Zubkov.