WASHINGTON (CNN) -- World powers are discussing next steps toward Iran if it fails to meet a year-end deadline for addressing international concern over its nuclear program, the White House and State Department said Tuesday.

Top officials from the so-called P5 plus one -- Britain, China, France, Germany, Russia and the United States -- held a conference call Tuesday to discuss possible sanctions against Iran, Assistant Secretary of State P.J. Crowley told reporters.

He said the group "united its resolve that Iran must either answer the questions that we have about its nuclear aspirations or face additional pressure" and that Washington would be "consulting broadly across the international community in the coming days and weeks" about its options.

In October the six nations offered Iran a deal to send most of its low-enriched uranium abroad for conversion into fuel for a medical reactor in Tehran.

White House spokesman Robert Gibbs told reporters that time is running out for Iran to accept the deal, noting the major international powers that offered the deal are urging Iran to accept it.

"The decision for them to live up to their responsibilities is their decision," Gibbs said of Iranian officials. "We have offered them a different path. If they decide not to take it, then the (major powers) will move accordingly."

Iranian President Mahmoud Ahmadenijad dismissed the offer Tuesday, telling Iranians in a speech in the southern city of Shiraz that the international community can give Iran "as many deadlines as they want, we don't care." He also accused the United States of fabricating a document said to detail Iranian plans for critical components of a nuclear device.

Gibbs countered his defiance, saying the international community is prepared to take additional steps if the year-end deadline comes and goes without any Iranian action.

"Mr. Ahmadinejad may not recognize, for whatever reason, the deadline that looms, but that is a very real deadline to the international community," Gibbs said.