Iran poses no immediate threat: Trump

US president Donald Trump says Iran constitutes no imminent threat, even as administration officials are poised to make the case to Congress that such a threat exists.

"We have no indication that anything has happened or will happen," Trump said at the White House last night, ahead of a political rally in Pennsylvania.

"I think Iran would be making a very big mistake if they did anything. If they do something, it will be met with great force. But we have no indication that they will," Trump said.

Administration officials plan to brief the entire House of Representatives and Senate in classified briefings today on their reasons for issuing undefined warnings two weeks ago about possible threats from Iran to oil shipping and infrastructure in the Middle East and to US interests in Iraq.

The reaction from senior lawmakers, who last week already received a briefing from US officials, has fallen along partisan lines. Democrats say they fear Trump's administration is setting the US on a course that will end with another major military conflict in the Middle East.

Trump yesterday again left open the possibility of negotiating with Tehran while dismissing reports of a behind-the-scenes mediation effort already underway.

"If they called, we would certainly negotiate. But that is going to be up to them. I would only want them to call if they are ready."

The US has set a 12-point ultimatum for Iran, demanding drastic changes in domestic and foreign policies, with secretary of state Mike Pompeo insisting that any talks between the two countries would have to be based on Tehran accepting those demands.