The United Nations nuclear watchdog chief demanded on Monday that Iran give U.N. inspectors “without further delay” access to a disputed military site where they believe Iran ran may have conducted tests tied to the development of nuclear weapons.

Expressing frustration over Iran's lack of cooperation, Yukiya Amano, director general of the International Atomic Energy Agency, said his agency needs to assess the “activities” at Iran's Parchin military base.

Iran says the charge of nuclear weapons development at the site is “baseless.” Tehran says its nuclear program is for peaceful use.

A U.S. research organization obtained satellite images in May that indicated Iran may have attempted to destroy traces of nuclear weapons testing at Parchin.

Despite the U.N.'s demands for access, Amano said “no concrete results” have come out of meetings since January with Iranian representatives

The stalemate comes as European nations said last week they are considering additional sanctions over Iran's nuclear program.

Iran on Sunday lashed out against talk of European Union sanctions, calling it “irresponsible.”

Foreign ministry spokesman Ramin Mehmanparast said in a statement that Western sanctions are “ineffective” and “obsolete.”

The White House said last week that Israel and the United States are committed to preventing Iran from obtaining a nuclear weapon, but there is still time to pursue diplomacy.

“I can tell you that there is no daylight between the United States and Israel when it comes to the absolute need to prevent Iran from acquiring nuclear weapons,” White House spokesman Jay Carney told VOA.

“That is the president's policy and he will use all aspects of American power towards the goal of preventing Iran from acquiring a nuclear weapon,” he said.