The international dispute over Iran's nuclear programme is entering a "critical phase", the head of a United Nations watchdog warned.
Mohamed ElBaradei, director general of the International Atomic Energy Agency, urged Tehran to take advantage of a "window of opportunity" to defuse the looming crisis before it escalated further.
Mr ElBaradei was speaking in Vienna at the start of a two-day emergency meeting of the IAEA board which was called to consider reporting Iran to the UN Security Council - a process which could eventually lead to international sanctions.
The move, which has the backing of the five permanent Security Council members - Britain, the United States, France, Russia and China - is expected to be approved overwhelmingly by the 35-nation IAEA board.
However, Russia and China have made clear that there should be no action in the Security Council before Mr ElBaradei had delivered his report on Iran to the next IAEA board meeting in March.
Mr ElBaradei said that the delay offered a "window of opportunity" for Iran to resume its freeze on uranium enrichment activity in compliance with its international obligations. "We are reaching a critical phase, but it is not a crisis," he said.
The Iranians continued to voice defiance, with chief negotiator Ali Larijani again warning they would move to large-scale enrichment and stop further snap inspections by the IAEA if they were referred to the Security Council.
In Washington, United States National Intelligence Director John Negroponte said that Iran probably did not yet have nuclear weapons, nor the fissile material needed for producing them.
"Nevertheless, the danger that it will acquire a nuclear weapon and the ability to integrate it with the ballistic missiles Iran already possesses is a reason for immediate concern," he told the Senate Intelligence Committee.
The IAEA board meeting will resume on Friday in Vienna.