Rubio welcomed the Europeans' "snapback" of sanctions, long pushed by President Donald Trump, but said: "At the same time, the United States remains available for direct engagement with Iran -- in furtherance of a peaceful, enduring resolution to the Iran nuclear issue."

"Snapback does not contradict our earnest readiness for diplomacy, it only enhances it," Rubio said in a statement.

"I urge Iranian leaders to take the immediate steps necessary to ensure that their nation will never obtain a nuclear weapon; to walk the path of peace; and to, by extension, advance prosperity for the Iranian people."

France, Britain and Germany earlier Thursday triggered a mechanism to reimpose United Nations sanctions on Iran for failing to comply with commitments over its nuclear program -- a step that was a top goal for Trump in his first term and which caused major friction between the United States and Europe.

Trump in his second term has swung sharply in different directions on Iran, insisting he sought a negotiated settlement but then ordering US air strikes on nuclear sites in support of an Israeli military campaign.