Washington DC - Donald Trump traded threats of war and annihilation with Iran amid renewed questions over alleged plots to assassinate the US president.

"I have very firm instructions. Anything happens, they're going to wipe them off the face of this earth," Trump said in a NewsNation interview that aired Tuesday, in response to a question on Iran's supposed threats to his life.

Earlier Tuesday, Iranian General Abolfazl Shekarchi was quoted as saying Trump already knew Tehran would not hold back if Supreme Leader Ayatollah Ali Khamenei would be killed.

"Trump knows that if a hand of aggression is extended toward our leader, we will not only sever that hand, and this is not a mere slogan," Shekarchi was quoted as telling Iranian state media.

"But we will set their world on fire and leave them no safe haven in the region."

Trump issued a similar warning to Iran a year ago, shortly after returning to the White House, when he told reporters "if they do it, they get obliterated."

Iran is still reeling from some of the biggest anti-government protests since the Islamic revolution in 1979.

Human rights groups are working to confirm the number of people killed during the protests, which are reported to be in the thousands.

Iranians began holding mass demonstrations amid widespread dissatisfaction with the government and an economic crisis worsened by crippling US sanctions.

Trump repeatedly hinted at launching an all-out attack on Iran, before seeming to back down as the protests subsided and Tehran issued assurances that detained protesters would not be executed.