OPEC president Chakib Khelil says oil prices will keep on rising because of the falling value of the US dollar - and has once again ruled out a boost in output.

Khelil, who is also the Algerian oil minister, repeated in a posting to the website of Libya's state-run National Oil Corp recent statements that the market is well supplied.

"As interest rates and the value of the dollar go down, oil prices will go up for sure," Khelil said.

Opec, which supplies 40% of the world's crude, will not reconsider its output level before a meeting set for September, he said.

Crude oil futures doubled in a year to a record $135.09 a barrel on 22 May and US oil for July delivery is today around $133.46 a barrel. July Brent crude was trading around $132.80 after gaining yesterday for a third day.

A militant attack in Nigeria that disrupted supplies and speculation that fuel subsidies in Asian countries will continue to spur demand, were the latest market drivers.

On the positive supply side, Nigeria yesterday signed a $3.1 billion (£1.56 billion) deal with the local arm of Royal Dutch Shell to plug a funding gap in their joint venture projects, the country's state oil firm NPC said.