However, it was not clear if the administration can yet rally the support it needs for a fourth round of UN sanctions when it said China is unable to join the United States, Russia, Britain, France and Germany in Brussels on Friday.
Both Russia and China, which a US official said had a scheduling problem barring it from attending the six-power meeting, have been more reluctant than the other four powers about tightening sanctions.
The group is known as the P5-plus-1, or the permanent five veto-wielding members of the UN Security Council, plus Germany.
With a year-end deadline, the administration signaled Friday that time is running out for Iran to seize its offer of diplomatic engagement for resolving nuclear and other issues.
Following up on pessimistic comments she made last week, Secretary of State Hillary Clinton said Obama's engagement policy had yielded few, if any results.
"We have reached out. We have offered the opportunity to engage in meaningful, serious discussions with our Iranian counterparts. We have joined fully in the P5-plus-1 one process. We've been at the table," Clinton said.
"But I don't think anyone can doubt that our outreach has produced very little in terms of any kind of positive response from the Iranians," the chief US diplomat said.
If engagement fails under what is called a dual-track strategy, the United States will try to rally the international community to press Iran into changing course on its nuclear program, she recalled.
"And certainly additional pressure is going to be called for in order to do that," she said Monday during a press briefing with Spanish Foreign Minister Miguel Moratinos.
In an interview Friday with Al-Jazeera English channel, Clinton said the world community will now turn toward using "more pressure, like sanctions" against Iran to halt its nuclear program.
Clinton said Iran has taken actions that show little sign it will respond to Obama's efforts to engage them as it has failed to build confidence in recent months, including since an October 1 meeting with the P5-plus-1 in Geneva.
For example, she said Iran has balked at a US-backed International Atomic Energy Agency (IAEA) proposal to ship abroad low-grade nuclear fuel so it can be further enriched and returned to refuel a Tehran medical research reactor.
Also undermining international confidence, she said, is Iran's continued crackdown on peaceful opposition to Iran's disputed election in June that gave incumbent Mahmoud Ahmadinejad another term in office.
She said Iran also fanned fears about its intentions when it failed to come clean on a secret uranium enrichment plant near the holy city of Qom, and noted that Iran has subsequently announced plans for 10 to 20 new nuclear plants.
State Department spokesman Ian Kelly told reporters that a P5-plus-1 meeting "won't be possible this year" because of a scheduling difficulty, raising questions about how soon the six could agree on concerted action.
A State Department official later confirmed it was China that "couldn't come on December 18" for a political directors meeting in Brussels.
But the official also said plans were afoot for a P5-plus-1 teleconference call on Iran, probably by next week.
The US Congress sent Obama on Sunday a giant spending bill that also requires periodic reports on the status of diplomatic efforts to freeze Iran's nuclear program as well as on US and global sanctions on the Islamic republic.
