Venezuelan President Hugo Chavez has warned oil prices could double if the US attacks Iran.
Chavez (r) and Iran's Mahmoud Ahmadinejad are staunch US critics
Opening the summit of the Organisation of Petroleum Exporting Countries (Opec), he said said the price of crude could reach $150 or even $200 a barrel.
Oil has been hitting record peaks of well over $90 a barrel as markets believe Opec will not boost production.
The Opec summit in Saudi Arabia is only the organisation's third in 47 years.
Mr Chavez kicked off the summit with a blistering attack on the US.
"If the United States was mad enough to attack Iran or aggress Venezuela again the price of a barrel of oil could reach $150 or even $200," he said.
Mr Chavez called for unity in Opec ranks, invoking geopolitical reasons.
"The basis of all aggression is oil. It is the underlying reason," Mr Chavez said, referring to the war in Iraq and US threats against Iran over its nuclear programme.
"Today, Opec stands strong. It is stronger than it has ever been in the past. Opec should set itself up as an active geopolitical agent."
But Mr Chavez's remarks were counterbalanced by King Abdullah, the head of state of the host nation, Saudi Arabia, the largest oil producer in the world and a US ally.
"Those who want Opec to take advantage of its position are forgetting that Opec has always acted moderately and wisely," he said.
"Oil shouldn't be a tool for conflict, it should be a tool for development."
Officials from Opec member countries have already said that the summit will not be taking any decisions on oil production levels, prompting an increase in prices on Friday.
Oil reached a record high of $98.62 a barrel earlier this month
They say it will be for energy ministers meeting next month to decide whether to increase supplies to the world oil market in response to the current crisis.
A division emerged ahead of the summit between Saudi Arabia and Iran, BBC economics correspondent Andrew Walker says.
Iranian officials wanted the final communique to express concern about the weakness of the dollar, our correspondent says.
It is an issue because oil is priced in the US currency although as far as Opec is concerned the dollar's decline has been offset by the rising price of crude oil, he adds.
Saudi officials were against including any such remarks. One is reported to have warned that it could add to the pressure on the dollar.
Along with the dollar value, geopolitical tensions and concern about tight supplies have been the forces driving oil higher.