"It is a matter of concern that activities which have taken place since February 2012, at the location within the Parchin site ... will have an adverse impact on our ability to undertake effective verification there," Yukiya Amano, International Atomic Energy Agency director general, said in his opening statement at the agency's Board of Governors meeting in Vienna.
In Washington, meanwhile, the White House has denied there was friction between the United States and Israel over what it would take for the United States to launch a military strike against the Islamic republic.
Iran maintains its nuclear program is for civilian purposes, but many countries contend it is trying to develop nuclear weapons.
Asked about comments by Israeli Prime Minister Binyamin Netanyahu urging Washington to set a "clear red line" beyond which the United States would launch a military attack, White House spokesman Jay Carney said President Obama is committed to preventing Iran from acquiring a nuclear weapon and has shared his position with Israel's leaders.
"And that's why the president has pursued the policy that he has that has put together with our international partners unrelenting pressure on Iran to abide by its international obligations and to forsake its nuclear weapons ambitions," Carney said during his Monday media briefing.
Israeli officials said the window of opportunity for taking out Iranian nuclear facilities is closing.
U.S. officials believe there still is time for diplomacy and punitive sanctions to work, but "we've also made clear that the window of opportunity for reaching a solution by that means will not remain open indefinitely," Carney said, adding that "all options" remained on the table.
"The line is the president is committed to prevent Iran from acquiring a nuclear weapon," Carney said, "and he will use every tool in the arsenal of American power to achieve that goal."
In his remarks before the board in Vienna, Amano said no concrete results have been achieved so far despite more dialogue between the IAEA and Iran since the beginning of the year.
"This is frustrating because without Iran's full engagement we will not be able to start the process to resolve all outstanding issues, including those concerning possible military dimensions to its nuclear program," the agency chief said. "We consider it essential for Iran to engage with us without further delay on the substance of our concerns."