US Secretary of State John Kerry has called his Iranian opposite number to protest Tehran's latest round of ballistic missile tests, a fresh bone of contention between the two sides.
Iran and the US have no formal diplomatic ties, but Kerry and Foreign Minister Mohammad Javad Zarif built a close working relationship during negotiations for last year's nuclear accord between Tehran and world powers.
The US argues that a series of apparent missile tests breach the terms of a UN Security Council resolution and will result in new economic sanctions - either from Washington or the world body.
"If they are confirmed, if they're true, as I said yesterday, we'll take up the appropriate action inside the UN," State Department spokesman John Kirby warned yesterday.
"But I don't want to convey the impression that we are only looking at multilateral or UN possibilities in terms of measures to deal with it.
