Washington DC - President Donald Trump on Tuesday said he was planning to reinstate what he called the "maximum pressure" policy against Iran over allegations that it is trying to develop nuclear weapons.

However, Trump also said he hoped the policy would "hardly have to be used."

Trump made the comment as he signed a memorandum reimposing the tough policy of sanctions against Iran, similar to during his first term.

The memorandum instructs every department in the US government to design sanctions on Iran, especially in relation to nuclear activities, a White House aide told Trump at the signing ceremony.

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This will give Trump "all of the possible tools" to prevent Iran from being a "malign actor," the aide said.

Trump expressed some regret for the severe measures, saying, "This is one that I'm torn about. Everybody wants me to sign it. I'll do it. It's very tough on Iran."

"Hopefully, I'm not going to have to use it very much," he said. "I'm unhappy to do it, but I really have not so much choice because we have to be strong."

"We will see whether or not we can arrange. We'll work out a deal with Iran and everybody can live together," he said.

Trump also announced that if he were assassinated by Iran, the country would be "obliterated."

"I've left instructions if they do it, they get obliterated, there won't be anything left," he said.