"We're not going to be making any nuclear dust in Iran," Trump said in Qatar, the second stop of his multi-day Gulf tour.

"I think we're getting close to maybe doing a deal without having to do this."

Oil prices immediately dropped on the comments by Trump, who said he was basing his optimism on new statements by Iran.

"You probably read today the story about Iran. It's sort of agreed to the terms," Trump said.

The president did not specify which remarks he was referring to, but an adviser to Iran's supreme leader Ayatollah Ali Khamenei, Ali Shamkhani, said in a US media interview that Tehran was willing to accept far-reaching curbs on its nuclear programme.

Shamkhani told ABC News that Iran would give up stockpiles of highly enriched uranium as part of a deal in which the United States lifts sanctions.

Trump said that Iran should "say a big thank you" to Qatar's emir, Sheikh Tamim bin Hamad Al Thani, who had pressed the US leader to avoid military action against his country's giant neighbour.

"Iran is very lucky to have the emir because he's actually fighting for them. He doesn't want us to do a vicious blow to Iran," Trump said.

Trump has pursued diplomacy with Iran as he seeks to stave off a threatened military strike by Israel on Tehran's contested nuclear programme.