The US would seek sanctions if Iran does not comply and has not ruled out military action. But Russia and China seek a softer line.
The diplomat said the powers faced a choice of whether "to go forward based on Iran’s answer by then or to make the tough decision on going back to the UN Security Council" for action against Tehran if it rejects the offer.
"There is a speeding up of meetings and deliberation in order to put pressure on Iran, clearly," the diplomat added.
Some nuclear experts and diplomats say the time has come to strike a compromise on Iran’s uranium enrichment work, such as keeping machines running on standby, in order to keep the crisis from spinning out of control.
But the US and Iran remain in complete disagreement, with Iran insisting on enriching and Washington saying the work must stop completely.