The White House on Saturday expressed unhappiness over Iran's announcement that it is ready to exchange uranium for nuclear fuel.
A senior Obama administration official said that proposal is inconsistent with the UN plan.
The official, who spoke on condition of anonymity because of the sensitive nature of the issue, urged Iran to accept the UN offer to help build confidence in claims that its nuclear program is for peaceful purposes.
Such a deal would meet a key demand of a UN-sponsored effort to defuse fears over Iran's nuclear ambitions. But instead of shipping most of its uranium abroad for processing before it is returned to Teheran, Iran's foreign minister said Saturday that the material instead would be exchanged in batches.