Vienna, Austria ? Iran threatened to retaliate Thursday in the face of almost certain referral to the U.N. Security Council for its nuclear activities, and the head of the International Atomic Energy Agency said the dispute was “reaching a critical phase.”

Ahead of a decision by the IAEA’s 35-nation board, U.S. and European delegates turned to behind-the-scenes diplomacy to build the broadest possible support for reporting Iran to the council over concerns it is seeking nuclear weapons.

Iran’s chief nuclear negotiator, Ali Larijani, warned IAEA chief Mohamed ElBaradei in a letter that referral would leave Iran no choice but “to suspend all the voluntary measures and extra cooperation” with the IAEA – shorthand for reducing IAEA monitoring to a minimum.

Cuba, Venezuela, Syria and a few other nations at odds with Washington remained opposed to referral. India was said to be leaning toward supporting referral.

Diplomats accredited to the IAEA meeting said backing for Iran had shrunk among the U.N. nuclear watchdog’s board since Russia and China swung their support behind referral at an overnight meeting with the United States, France and Britain – the other three permanent council members – that started Monday.

“There’s a solid majority in favor of reporting,” said Gregory Schulte, chief U.S. delegate to the IAEA. “There’s even a more solid majority after Monday.”

Iran remained defiant. Larijani told ElBaradei that his country would severely curtail agency inspections and resume uranium enrichment if reported to the council.

Furthermore, “all the peaceful nuclear activities being under voluntary suspension would be resumed without any restriction,” he wrote in the letter, suggesting a resumption of work on full-scale uranium enrichment – a possible pathway to nuclear arms.

ElBaradei said there was a “window of opportunity” to defuse the crisis, stressing that even if the issue is referred, the Security Council would not take up the issue before next month.

“We are reaching a critical phase, but it is not a crisis,” he said.”