Iran faces "further measures" such as sanctions if it does not respond positively to an offer by major powers to suspend its nuclear programme for incentives, the US State Department said.
"We agreed in the absence of a clear, positive response from Iran that we have no choice but to pursue further measures against Iran as part of this strategy," said State Department spokesman Gonzalo Gallegos to Reuters.
He added that Tehran said it would provide a written reply to the offer tomorrow.
Earlier today a conversation between Iran’s nuclear negotiator Saeed Jalili and EU foreign policy chief Javier Solana, regarding the offer, ended inconclusively.
"The phone conversation has not been conclusive. We do not exclude new contacts in the coming days," an EU official told Reuters after the conversation.