The United States warned Iran that December is "a very real deadline" after Iranian President Mahmoud Ahmadinejad dismissed an international ultimatum over its nuclear program.
The United States and France have repeatedly urged Tehran to accept a UN nuclear watchdog-drafted deal to swap enriched uranium for nuclear fuel by the end of the year or face the threat of further sanctions.
White House spokesman Robert Gibbs said that the so-called P5+1, which gathers UN Security Council veto-wielding permanent members Britain, China, France, Russia and the United States plus Germany, were all on board with the deadline.
"I think that the international community is united in this. This is not (just) something that the (US) president has said... This is something that the members of the P5+1 have said," Gibbs said.
"That's why we are at the point where we are now with the international community waiting to see and have been waiting to see for months whether Iran will live up to its responsibilities."
Ahmadinejad earlier rejected the deadline and delivered another broadside against Western pressure over Tehran's refusal to suspend enrichment or agree to full inspections by the International Atomic Energy Agency (IAEA), the UN nuclear watchdog.
"They say we have given Iran until the end of the Christian year. Who are they anyway? It is we who have given them an opportunity," Ahmadinejad said in a speech in the southern Iranian city of Shiraz.
Gibbs said: "Mr Ahmadinejad may not recognize, for whatever reason, the deadline that looms, but that is a very real deadline for the international community... It is in his control what Iran decides to do."
US President Barack Obama's administration has signaled that time is running out for Iran to seize its offer of diplomatic engagement for resolving nuclear and other issues.
"As we have said many times, the president has stressed that we and our partners will be assessing Iran's responsiveness here as we approach the end of the year," said State Department spokesman Philip Crowley.
He warned that, "come 2010, should Iran continue in its current posture... there will be implications and consequences for their failure to take advantage of this opportunity."
A senior diplomatic official who spoke to AFP on condition of anonymity said: "These things do take time... we have come into office with a sense of urgency and we have been working steadily on this issue."
The official said the administration would carefully seek consensus before acting.
"When we pull the trigger, we want to be sure that there's a broad understanding among those who play a role in the Security Council as to what we should be doing."
The United States has raised the specter of a fourth round of UN sanctions, but will need to persuade Russia and China to drop their traditional reluctance to consider tougher measures.
Iran insists its nuclear program is solely for civilian purposes and rejects Western suspicions that it is covertly trying to develop a bomb.
Its top nuclear negotiator, Saeed Jalili, told Japanese Prime Minister Yukio Hatoyama on Monday that weapons of mass destruction "are against Islamic teaching and that Iran would never develop such weapons," according to Hatoyama's office.
"We seriously oppose mass destruction nuclear weapons," Jalili told reporters, rejecting as "baseless" reports that a plane-load of arms from North Korea seized in Thailand was headed for Iran.
Thai officials said they impounded the plane on a US tip after it landed to refuel at a Bangkok airport on December 11 with cargo that included shoulder-launched missiles and rocket-propelled grenades.
Iran has rejected the US-backed IAEA proposal to ship low-grade nuclear fuel abroad so it can be further enriched and returned to refuel a Tehran medical research reactor.
But Ahmadinejad told AFP in an interview on Friday that Iran is ready to strike a uranium enrichment deal if the United States and the West respect the Islamic Republic and stop making threats.