Sanctions on Iran by the OECD states could push oil prices up 20-30 percent to $140 per barrel unless alternative supplies from developing countries come on line, the International Monetary Fund said on Thursday.

"A halt in Iran's exports to OECD economies without offset from other sources would likely trigger an initial oil price increase of around 20-30 percent, with other producers or emergency stock releases likely providing some offset over time," the IMF said in its report for the G20.

The EU voted on Monday to ban oil imports from Iran. The move came after the Islamic Republic announced earlier this month that it had launched a nuclear enrichment program at a well-protected underground facility near the holy Shia city of Qom.

Western nations suspect Iran, which is already under numerous international sanctions, of pursuing a secret nuclear weapons program but Tehran insists it needs nuclear power solely for civilian purposes.

Iran has threatened to retaliate by blocking the Strait of Hormuz, the main export route for supplies from the Middle East.

"(The Strait's closure) could trigger a much larger price spike including by limiting offsetting supplies from other producers in the region," the IMF added.