Iranian President Mahmoud Ahmadinejad asked Iran's atomic chief yesterday to enrich uranium to 20 percent, in a fresh challenge to world powers, days after appearing to agree to a deal with Western powers to swap enriched uranium for nuclear fuel.

Ahmadinejad blamed the world powers for the stalemate over an UN-drafted nuclear fuel deal, but left the door open for possible negotiations over the proposal.

He also insisted that world powers "unconditionally" accept exchanging Iran's low-enriched uranium (LEU) for high purity 20 percent enriched uranium, which Iran says is required as nuclear fuel for the Tehran reactor which makes medical isotopes.