World powers - the five permanent members of the U.N. Security Council plus Germany - are pressuring Iran for "urgent" access to sites involving its controversial nuclear program.
With talks between the two sides set for mid-month, the powers are seeking to assess the possible military capability of the sites. A joint statement issued by the six world powers on Thursday, following a meeting in Vienna, said Iran must take "concrete steps" to resolve the dispute.
The International Atomic Energy Agency has repeatedly asked Iran for access to the Parchin military complex near Tehran to investigate allegations of military-related nuclear research at the site. Iran has refused that request.
Iran and the six world powers resumed talks on the dispute last month in Istanbul after a more than year-long stalemate. The next round of negotiations is due to take place in Baghdad on May 23.
Iranian officials said they are optimistic about making progress in the talks but will not give up what they see as their right to the peaceful use of nuclear energy.
Iranian Deputy Foreign Minister Mohammad Majdi Akhondzadeh said Wednesday the gravest threat to Middle East stability and security is the undeclared Israeli nuclear weapons program. Israel is widely assumed to be the only nuclear-armed nation in the region but neither confirms nor denies possessing atomic bombs. Akhondzadeh was speaking in Vienna at an international meeting to review the nuclear Non-Proliferation Treaty.
Israel sees a nuclear-armed Iran as a threat to its existence because of Iranian leaders' repeated calls for the destruction of the Jewish state.
Some information for this report was provided by AP and Reuters.