Trump has given conflicting signals since announcing on Monday that he had called off renewed attacks to give time for negotiations, veering between optimism about an agreement and threats of more action.

"It's right on the borderline, believe me," Trump told reporters at Joint Base Andrews near Washington when asked where the talks with Iran stood.

"If we don't get the right answers, it goes very quickly. We're all ready to go. We have to get the right answers -- it would have to be a complete 100 percent good answers."

Trump said it would save "a lot of time, energy and lives" if Iran made a deal, saying it could happen "very quickly, or (in) a few days."

The US leader said this week he had been an hour away from ordering the resumption of strikes on Iran but postponed the attack planned for Tuesday at the request of Gulf states.