"I expect in the next few weeks (for Iran) to come and sign the protocol," International Atomic Energy Agency (IAEA) director general Mohamed ElBaradei told reporters in an interview at the agency's headquarters in Vienna.
"They haven't communicated a date yet, but we understand it will be shortly."
Hassan Rowhani, who handles Iran's nuclear affairs as secretary of the Islamic republic's Supreme National Security Council, said last week that "it won't be very long" before Tehran signs the additional NPT protocol.
The IAEA last week condemned Iran for 18 years of covert nuclear activities but stopped short of bowing to US demands of hauling Tehran in front of the UN Security Council for possible sanctions.
ElBaradei is due to file a report in February on the full scale of Iran's nuclear activities, for which Iran has pledged to supply all information required by the IAEA.
Even with Iran's recent pledges of cooperating with the IAEA, ElBaradei warned that the international community would have to wait a while before it can be certain about the full-scale of Iran's nuclear activities.
"It's a lot of work... before we can come to a conclusion that the Iranian (nuclear) programme has been fully declared," ElBaradei said.