State Department spokesman Richard Boucher's comments seemed designed to pressure Greek and Turkish Cypriot leaders who have rejected the plan and to appeal over their heads to the Cypriot people to grasp what US officials call the chance of a lifetime.
"The plan was the culmination of talks between Greek- and Turkish-Cypriots, as well as Greece and Turkey. It is the only plan. It is the final plan," Boucher told reporters on Monday.
"There's been speculation among Cypriots that there could be some other alternative to this plan. In our view, there are no options ... so the vote turns out to be this settlement or no settlement," he added.
President George Bush spoke earlier on Monday to Greek Prime Minister Costas Karamanlis and they both "agreed on the importance to reach a Cyprus settlement," White House spokesman Scott McClellan told reporters aboard Air Force One.
"I think everybody's estimate is that there's not a lot of prospects for reaching any other deal, other than this deal"
US State Department
Pressed on whether the United States would abandon any effort to resolve the matter, Boucher softened his stance and said: "We can't say never and ever."
But he then added. "There is no plan B. There is no alternative diplomatic course. There's no promise that if this gets voted down, we'll go back the next day and try again."
"I think everybody's estimate is that there's not a lot of prospects for reaching any other deal, other than this deal," he said.
Cyprus has been divided along ethnic lines since 1974, when Turkey occupied the north of the Mediterranean island after a brief coup in Nicosia, engineered by the military junta then ruling Greece.
The plan, the final details of which were decided by UN Secretary-General Kofi Annan after the two sides failed to negotiate an agreement, proposes re-linking Cyprus under a loose federation of two largely self-governing states.
It will be put to a vote in dual referendums on 24 April, in time for the entire island to join the European Union on 1 May, if both vote "yes." If either votes "no," only the Greek Cypriot south will join, deepening the isolation of Turkish Cypriots and harming Turkey's effort to join the EU.