The head of the U.N. nuclear watchdog pressed Iran on Monday to grant his inspectors immediate access to the Parchin military site, where they believe Tehran may have conducted explosives tests relevant to the development of nuclear weapons.

Yukiya Amano, director general of the International Atomic Energy Agency, also said it was “frustrating” that the IAEA and Iran had made no concrete progress in talks that began in January aimed at allaying concern about suspected atom bomb research.

Western powers may seize on his statement to a closed-door session of the IAEA’s 35-nation governing board to strengthen their case for further international pressure on Tehran, one of the world’s largest oil producers.

Amano made his comments one day after Israeli Prime Minister Benjamin Netanyahu said that Israel and the United States were in discussions on setting a “red line” for Iran’s nuclear work, which Tehran says is entirely peaceful.

Israel, believed to be the only nuclear-armed state in the Middle East, sees the possibility of Iran developing an atomic bomb as a threat to its existence and has said it may use military means if diplomacy and sanctions fail.

European Union heavyweights Britain, France and Germany called last week for new sanctions and Canada has unexpectedly severed ties with Iran.

“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,” Amano said, according to a copy of his speech.

Iran’s envoy to the IAEA, Ali Asghar Soltanieh, told reporters that Tehran would “continue” to cooperate with the U.N. agency but that its national security must be taken into consideration.

He said discussions were under way this week about the possibility of having another meeting with the IAEA. The U.N. agency was not immediately available for comment.