"I think it was intentionally ambiguous... We'll have to wait and see what Iran's actions actually are" before deciding the US response, Pompeo said after a meeting with British foreign minister Jeremy Hunt.

"They've made a number of statements on actions they intend to do in order to get the world to jump. We'll see what they actually do. The United States will wait to observe that and, when we do, we'll make good decisions," he said.

"I'm confident as we watch Iran's activity, that the United Kingdom and our European partners will move forward together to ensure that Iran has no pathway to a nuclear weapons system," he said.

Pompeo also raised the subject of Instex, a trade mechanism launched by Britain, France and Germany earlier this year in a bid to allow Tehran to keep trading with EU companies bypassing US sanctions.

"We talked to UK, Germany and France about Instex. There are provisions in the sanctions we put in place that allow humanitarian aid and certain products to get into the country," he said.

"We said so long as that vehicle is being used for that limited purpose, non-sanction purpose, it's of course unobjectionable. When transactions move beyond that... we will evaluate, review it, and if appropriate there will be sanctions against those who were involved in that transaction," he added.