The US expressed "disappointment" with Iran following an international meeting on Teheran's nuclear program Friday, though it did not publicly move to stiffen sanctions.
The permanent members of the UN Security Council - the US, Britain, France, Russia and China - as well as Germany met at the foreign ministry staff level in Brussels to assess Iran's response to earlier agreements worked out, in which Iran would hand over some of its low-enriched uranium for processing overseas.
Since that framework was mapped in Geneva in October, however, Iran has declined to finalize the deal, setting new conditions on that and two other arrangements made then.
"We urge Iran to reconsider the opportunity offered by this agreement... and to engage seriously with us in dialogue and negotiations," the statement put out by the six world powers after Friday's parley read.
An EU official said there was no mention of imposing sanctions on Iran for its refusal to halt nuclear enrichment activities.
"These things are a matter of timing, and this was not the right time for it," said the official, who asked not to be named because of the sensitivity of the matter.
The meeting followed heightened rhetoric from the United States raising the prospect of additional sanctions, but on Friday US State Department spokesman Robert Wood also declined to spell out when that step might be taken.
"If it doesn't respond to the calls of the international community for it to live up to its international obligations, then we will have to look at the pressure track. But I don't want to get ahead of where we might go on that," he said of Iran and the possibility of future sanctions.
"We are disappointed by the lack of follow-up to the three understandings reached in Geneva," he added. "There's a window of opportunity for Iran. That window is not going to be open forever."
Wood indicated that a follow-up meeting in which the six countries at the talks would "take a closer look at what measures we may need to take with regard to Iran" would be held soon. But he added that no date had been set.
In response to a query questioning the idea of giving Iran more time, in light of the fact that it has refused to comply with international demands on its program to date, Wood replied, "We have said from the beginning, we're willing to go the extra mile with regard to diplomacy."
Wood also gave strong backing to a UN resolution condemning Iran for human rights violations following its violent crackdown on the opposition after June's contested presidential election.
"The United States welcomes the resolution passed today by the United Nations calling upon the government of Iran to respect its human rights obligations fully," he said.
He also pointed out that "this is the largest vote margin on such a resolution on Iran in the UN ever. Over 60 percent of those members voted in support."
Meanwhile, Iranian state TV reported that the country would begin large-scale air defense war games on Sunday, aimed at protecting its nuclear facilities against any possible attack.
Gen. Ahmad Mighani, who heads an air force unit dealing with threats to Iran's air space, said the war games would cover regions where the country's nuclear facilities were located, according to the report.
The official IRNA news agency said the drill would cover 600,000 square kilometers in central, western and southern Iran.
In related news, Iran's ambassador to the International Atomic Energy Agency said his nation needed enriched uranium to power some 200 hospitals, and insisted "guarantees" were needed before Teheran could consider a storage deal with Turkey.
Ali Asghar Soltanieh said in an interview with Der Spiegel published on Saturday that Iran desperately needed more uranium to produce isotopes for medical purposes.
Soltanieh was also quoted as saying Teheran would not allow Turkey to store its enriched uranium temporarily unless there were guarantees it would receive fuel in exchange.
AP contributed to this report.