Washington insists that Iran must turn over its near-weapons-grade enriched uranium, agree to curb its nuclear activities and re-open the Strait of Hormuz for any peace agreement to take hold.

"I think now, in some of the papers that have been exchanged back and forth, it's clearly addressed, but we...still don't have final sign off from their system as of this morning," Rubio told the House Foreign Affairs Committee.

Rubio also doubled down on his assertion that the war in Iran was over, even as Iran attacked Kuwait's airport, killing one and wounding 63 people on Wednesday, in a significant escalation.

"We're no longer conducting sustained strikes inside of Iran to degrade their military, because Epic Fury is over," Rubio told the panel, asserting that the United States has scored a victory.

"We define victory as destroying their defense industrial base, significantly reducing the number of missile launchers that they possess, significantly reducing their stockpile of drones," Rubio said. "And we achieved all those, in addition to destroying what they had left of an air force and wiping out their entire conventional navy."

Iran has said it needs the release of $12 billion in frozen assets before engaging in substantive talks on its nuclear program, and dismissed earlier comments by US President Donald Trump, suggesting that its stockpile of enriched uranium would ultimately be destroyed.

The war that began with US-Israeli strikes on Iran on February 28, has engulfed the entire Middle East, with Iran targeting US allies in the region and effectively blocking the vital oil shipping Strait of Hormuz.