WASHINGTON -- US President Donald Trump said Monday that any country doing business with Iran will pay a tariff rate of 25 percent on any business conducted with the United States.

The order is "effective immediately," Trump announced on Truth Social, adding that it is "final and conclusive."

The statement marks the latest escalation in Washington's economic pressure campaign against Iran, as Trump repeatedly threatened that his administration is weighing "some very strong options" including potential military action.

White House press secretary Karoline Leavitt said earlier Monday diplomacy remains the preferred approach for the United States to deal with Iran, though the Trump administration will not rule out military options if necessary.

"President Trump has always expressed that diplomacy is the first option," Leavitt said on Fox News. "However, he is unafraid to use the lethal force and might of the United States military if and when he deems that necessary."