World powers were close to overcoming their differences late on Wednesday on what message the UN atomic watchdog will send to Iran when its board of governors meets on Thursday, diplomats said.
Diplomats to the International Atomic Energy Agency (IAEA) in Vienna were "close but not there yet" to agreeing on a resolution amenable to all the main powers including Russia and China, one Western envoy said.
Another said they were "cautiously optimistic", although time differences meant that it would most likely not be until Thursday morning, just before the meeting starts, that a deal is reached.
"This is going to go right down to the wire," the envoy said.
"The resolution will call on Iran to intensify dialogue with the agency and comply fully with its obligations," the first diplomat said on condition of anonymity, calling the discussions "intense".
"It also calls on the director general (of the IAEA, Yukiya Amano) to report in March on the status of the resolution," the envoy added, saying diplomats were "guardedly optimistic" on reaching a deal.
Last week the IAEA came the closest yet to accusing Iran outright of seeking to develop nuclear weapons, in a hard-hitting report immediately rejected by Tehran as "baseless".
Iranian Foreign Minister Ali Akbar Salehi said on Wednesday Tehran would send "an analytical letter with logical and rational responses" to the agency, which it has accused of being "politicised".
Washington, Paris and London jumped on the IAEA report as justification to tighten the screws further on Iran, already under four rounds of Security Council sanctions and additional US and European Union restrictions.