Oil prices hit a high of $139.12 on Friday
Crude oil prices have risen $5 to more than $136 a barrel, because of concerns that a possible strike by oil workers in Nigeria may disrupt supply.
A union representing Chevron workers in Nigeria, Africa's biggest oil producer, has renewed a strike threat.
US light, sweet crude rose to $136.74 a barrel, while in London, Brent crude was $136.09 a barrel.
Oil prices have been particularly volatile recently. Earlier in the day, prices were close to $132 a barrel.
They fell after the US dollar strengthened against the euro, boosted by better-than-expected news from US retailers.
When the US currency strengthens the price of oil becomes more expensive to overseas investors.
Oil prices hit a high of $139.12 on Friday.
Despite the recent volatility, the future trend is for even higher prices, analysts say.
Since the start of the year, oil prices have risen by 60% and consumers worldwide have seen pump prices soar.
"It's going to be a painful summer for American drivers," said Eric Wittenauer, analyst at Wachovia Securities.
Carmaker BMW said on Thursday that production at one of its UK plants would have to be suspended over the weekend after the protests disrupted the supply of crucial parts.
There have been fuels protests in many countries, including Spain, Portugal, and Malaysia. Indian lorry drivers are threatening to strike next month.
The International Energy Agency (IEA) said: "The prices are not sustainable and jeopardise economic growth globally."
The IEA's executive director Nobuo Tanaka said the impact was "especially acute in developing countries".
The world's biggest producer, Saudi Arabia, has called an emergency oil meeting in Jeddah on 22 June to try to calm the volatile market.