Oil prices have risen as supply concerns in Nigeria and Iran overshadow a forthcoming meeting in Vienna of Opec ministers.
Militants are waging a violent campaign against Nigeria's oil industry
US crude rose 51 cents to $68.27 a barrel in out of hours trade. London's Brent crude rose 34 cents to $66.58.
Nigerian militants have pledged to wage a violent campaign against the nation's oil industry, despite the release of four foreign oil workers held hostage.
Uncertainty about the future of Iranian oil production also unnerved traders.
US and European officials are meeting in London on Monday in an effort to persuade Russia and China to back tough action against Iran if the country continues with its controversial nuclear programme.
Analysts fear Tehran could use its position as a major oil producer as a political weapon if the crisis over the programme, which Western nations say could lead to Iran developing nuclear weapons, escalates.
Oil prices have risen by more than $7 a barrel so far this year on growing fears over disruption to crude supplies.
Armed groups in Nigeria's key Niger Delta oil producing region say they plan to continue attacks against installations with the aim of reducing the country's crude exports by 30%.
Despite the uncertainty, members of the Opec oil producers' cartel are expected to agree to keep oil output at a near 25-year high when they meet in Vienna on Tuesday.
The US, which is the world's biggest oil consumer, has called on Opec nations to continue pumping oil at current levels of about 28 million barrels a day.
Speaking ahead of the meeting, Saudi Arabia's oil Minister Ali Al-Naimi said the world's biggest exporter saw "absolutely" no need for a cut in production this year.
Algerian energy and mining minister Chakib Khelil added: "I think we should leave things as they are."
But Venezuelan energy minister Rafael Ramirez said Opec members had to be prepared to consider a cut in production.
"The market looks well supplied, in fact it's oversupplied. We believe that Opec has to be ready to cut (production), maybe now at this meeting, maybe at the next meeting," Mr Ramirez said.
"We believe that if the United States insists on putting pressure on Iran, the price will be maintained at the level we are seeing," he added.