The chief United Nations nuclear inspector says no deal has been reached with Iran to investigate its controversial nuclear program, but that a meeting will take place next month.

International Atomic Energy Agency team leader Herman Nackaerts and his team of U.N. inspectors arrived Friday at Vienna airport following two days of “intensive discussions” in Tehran.

Citing unspecified differences, Nackaerts said they were unable to produce an agreement on access to Iranian nuclear facilities.

“We could not finalize the structured approach to resolve the outstanding issues regarding possible military dimensions of Iran's nuclear program.”

Nackaerts said another round of talks will take place on February 12 in Tehran.

The IAEA had hoped to gain access to the Parchin military site, which Western nations suspect is related to nuclear weapons development. Iran says it is a conventional military site and that its nuclear program is strictly for peaceful purposes.

Iranian Foreign Ministry spokesman Ramin Mehmanparast said Tuesday that a religious decree issued by Iran's supreme leader banning nuclear weapons is binding on the Iranian government.

The IAEA visit comes as international diplomats are again setting the stage for separate negotiations with Tehran over curbs to its nuclear ambitions.

Iran and the so-called P5+1 contact group – the United States, Britain, Russia, China, France and Germany – are expected to try to open talks in the coming weeks after a seven-month hiatus.

The last time Iran's nuclear negotiators met with their foreign counterparts, in Moscow in June, the talks did not go well. Both sides wanted their maximum demands met, and they offered little in return.