New Delhi, May 21 -- Donald Trump warned on Wednesday that talks with Iran were balanced on a knife's edge, telling reporters the situation was "right on the borderline" between a negotiated peace deal and a fresh wave of US military strikes, even as he signalled Washington was prepared to wait a few more days for Tehran to respond.

Speaking to reporters at Joint Base Andrews near Washington, Trump delivered some of his starkest language yet on the state of negotiations with Iran, leaving little ambiguity about what a breakdown in talks would mean.

"It's right on the borderline, believe me," Trump said. "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 ...