Iran formally submitted its response to the latest American peace proposal on Sunday evening, handing the document to Pakistani mediators.
The move puts the fate of a fragile, open-ended ceasefire squarely back in Washington's hands.
US President Donald Trump continued with his verbal attacks and warnings on Sunday, saying that the US is directly monitoring Iran's enriched uranium stockpile and will act if anyone approaches it.
A Situation Room meeting with senior national security officials is expected at the White House on Monday, where the administration will assess Iran's submission and decide next steps.
The ceasefire itself has no fixed expiry date, and Trump must decide whether to keep talking or follow through on his repeated threat to resume bombing "at a much higher level and intensity than before."
Tehran's counter-proposal centres on two demands: an immediate and permanent end to hostilities, and restoration of maritime security in the Persian Gulf and the Strait of Hormuz.
Violence persists at sea, with a drone striking a cargo vessel in Qatari territorial waters on Sunday morning, sparking a brief fire onboard.
