Speaking after negotiators from Iran and the international group resumed talks in Vienna, National Security Council spokesman John Kirby told reporters there is "a deal on the table" and the Iranians "ought to take it."

"You've heard the president say we're not going to wait forever for Iran to take this deal," Kirby said.

"I'm not going to slap a label on it and say 'last ditch,' but clearly time does appear to be getting very short in terms of being able to get to a deal. And again, we urge Iran to take this deal on the table," he said.

The proposed deal would see Iran agree to stringent controls on its nuclear sector, which it insists has only civilian purposes but is suspected of harboring a secret military program.

In return, Iran would see a gradual lifting of crippling economic sanctions.