BERLIN (AP) — The head of the U.N.'s atomic watchdog agency will head to Tehran next week to press Iranian authorities for access to sites where the country is thought to have stored or used undeclared nuclear material, the organization said Saturday.

It will be the first visit to Iran of International Atomic Energy Agency Director General Rafael Grossi since he took office last December, and comes amid intense international pressure on the country over its nuclear program.

The focus will be on access to sites thought to be from the early 2000s, before Iran signed the 2015 nuclear deal with world powers. Iran maintains the IAEA inspectors have no legal basis to inspect the sites.

“My objective is that my meetings in Tehran will lead to concrete progress in addressing the outstanding questions that the agency has related to safeguards in Iran and, in particular, to resolve the issue of access,” Grossi said in a statement.

“I also hope to establish a fruitful and cooperative channel of direct dialogue with the Iranian government which will be valuable now and in the future.”

The Iranian delegation to international organizations in Vienna tweeted that “we hope this visit will lead to reinforced mutual cooperation.”

Since President Donald Trump unilaterally pulled the U.S. out of the nuclear deal with Iran in 2018, the other countries involved — France, Britain, Germany, Russia and China — have been struggling to keep it alive.

The deal, known as the Joint Comprehensive Plan of Action, or JCPOA, promises Iran economic incentives in exchange for curbs on its nuclear program. But with the reinstatement of some American sanctions, Iran's economy has been steadily deteriorating and Tehran has begun violating provisions of the agreement to try to pressure the other countries to do more to offset those sanctions.

At the same time, Iran has continued to provide IAEA inspectors woth access to its nuclear facilities — one of the major reasons the countries still party to the agreement stress it's important to keep it alive.

Last week, the U.S. ratcheted up the pressure, officially informing the United Nations it was demanding the restoration of all U.N. sanctions on Iran, arguing that Iran is in non-compliance and invoking a provision of the nuclear deal to “snap back” even more sanctions.

Russia, China, Britain, France and Germany, who often disagree, all declared the U.S. action illegal, arguing it is impossible to withdraw from a deal and then use the resolution that endorsed it to re-impose sanctions.

Iran has also rejected the move, but the U.S. has stuck to its guns, declaring that a 30-day countdown for the snapback of penalties eased after the 2015 deal was signed had begun.

The five nations and Iran are due to meet in Vienna on Sept. 1.