"Right now, it hasn't been very productive because the Iranian demands are simply not just unacceptable, but in some cases … really not realistic," Rubio told reporters in Florida before departing for France to attend a Group of Seven (G7) foreign ministers' meeting.

"I wouldn't say we're at the verge of a breakthrough," said Rubio, reiterating that the Trump administration preferred a diplomatic solution to the conflict.

In an interview earlier on Sunday, U.S. President Donald Trump told CNN that it is "highly unlikely" for the United States to resume large-scale military strikes against Iran as long as negotiations make progress. He did not rule out the possibility of further military action.

The U.S. Central Command announced on Saturday the formation of Task Force Iron Gate, a unit of about 20 military and civilian personnel tasked with coordinating efforts across U.S. government agencies to disrupt the movement of advanced weapons and other materials across Iran's land borders and other routes.

Trump said on social media Sunday that the task force was "in full force and effect" in Iran, and that, combined with the U.S. naval blockade, "very little gets into, or out of, Iran."