WASHINGTON, Aug 3 (Reuters) - U.S. President Donald Trump on Monday said Iran faced "decapitation" if it did not agree a pact to end the conflict between the two nations, saying Tehran had one last chance to strike a deal.

"I think we're going to maybe get something, but I want to give them every last chance before decapitation," he said, reiterating a threat to launch a major attack on Iran.

Earlier on Monday, after Iran said there were no talks under way, Trump slammed what he called Iran's "unbelievably duplicitous" leadership.

Trump, asked about the status of the negotiations, told reporters "they are going on right now," adding that the two sides were speaking at the request of Iran, Saudi Arabia, the United Arab Emirates and Qatar among others.

"This is a last chance for them to sign a good document," he said, referring to Iran.

(Reporting by Nandita Bose, writing by David Ljunggren; editing by Michelle Nichols)