The International Atomic Energy Agency began a new round of talks with Iran in an effort to gain access to sites where suspected nuclear weapons research has been conducted.
Tehran's envoy to the IAEA met with agency officials Friday in Vienna.
The world nuclear watchdog wants access to Iran's Parchin military base, where the agency believes Iran has been conducting atomic weapons research.
Both sides say progress had been made in previous discussions on the issue this year, but no deal acceptable to both parties had been reached.
Tehran says Parchin is a conventional weapons facility, and insists the Iranian nuclear program is peaceful.
However, satellite photographs of the site taken on May 25 show two buildings at Parchin the IAEA had sought access to had been razed over by Iran.
“These activities raise further concerns of Iranian efforts to destroy evidence of alleged past nuclear weaponization,” the Washington-based ISIS said in a six-page written analysis.
Iran and the P5+1 - the five permanent members of the UN Security Council plus Germany - have held two failed rounds of negotiations aimed at halting Iran's controversial uranium enrichment program.
In Beijing on Friday, President Hu Jintao urged visiting Iranian president, Mahmoud Ahmadinejad, to be “flexible and pragmatic” at the third round of talks scheduled for Moscow on June 18 and 19, and to cooperate with the IAEA.