Speaking to reporters on Friday, the head of the International Atomic Energy Agency (IAEA) said the agency would be keeping a close eye on Iran’s nuclear programme.
"We are going to adopt a very vigorous approach, a very intensive approach to try to complete our work," IAEA director general Muhammad Al-Baraday said.
Al-Baraday said the IAEA ultimatum was "sending a very powerful message to Iran that they need to cooperate fully and immediately and to show complete transparency".
"We also need the support of other countries who have been supporting the Iran (nuclear) programme," Al-Baraday said.
The IAEA needed to make sure it obtained "all the information we need", he added.
Earlier, Iranian officials against any form of an ultimatum threatened to cease cooperation with the IAEA. But al-Baraday responded by saying: "I think they will get over that. I think they will understand it is in their interest to cooperate with us."
"I need them to be pro-active, I need them to be transparent," Al-Baraday said.
"We are not going to duck any issue but we are not going to jump to any conclusions," he added.