Oil prices have continued to rise on Friday, pushed higher by fears that production facilities may be attacked in Nigeria and Saudi Arabia.
Militants have threatened a crippling blow to Nigeria's oil output
Nigerian production has been disrupted by militants, while Saudi Arabia said it halted a plan to bomb a local plant.
There also are concerns that a European and US row with Iran over its nuclear programme may crimp oil exports.
US light crude closed up 31 cents to $63.67 a barrel, while Brent crude ended up 11 cents to $64.18.
Although anti-government militants in Nigeria's Niger Delta region have released six of the nine foreign oil workers they recently kidnapped, they have pledged to deal "one huge crippling blow to the Nigerian oil industry".
"We will commence with attacks in another area of the Niger Delta with an aim to ensuring the total discontinuation of export of onshore crude oil," the militants said in a statement.
Analysts such as Kevin Norrish of Barclay Capital said the violence in Nigeria had "the potential to get a lot worse".
"It's going to run and run," he said.
In addition to the continuing row over Iran, there is also the threat of attacks on oil facilities in the world's largest producer, Saudi Arabia.
Saudi security forces on Monday killed five suspected militants believed to be planning to attack an oil facility.
Iran has been threatened with sanctions over its decision to restart its controversial nuclear programme and Tehran has warned it would retaliate by cutting crude exports.
The Opec group of oil exporters meets next week to consider changing its current export levels.