US President Donald Trump on Monday said he wants to impose sanctions on Iran's oil gradually, citing concerns about shocking energy markets and causing global price spikes.

"With the oil, it's very interesting. We have the toughest sanctions ever imposed, but on oil we want to go a little bit slower because I don't want to drive (up) the oil prices in the world," he told reporters before flying to a campaign event.

 "This has nothing to do with Iran ... I could get the Iran oil down to zero immediately but it would cause a shock to the market. I don't want to lift oil prices."