International Atomic Energy Agency (IAEA) Director General Yukiya Amano smiles as he waits for a board of governors meeting to begin at the IAEA headquarters in Vienna, Austria June 6, 2016. REUTERS/Heinz-Peter Bader

VIENNA (Reuters) - The head of the atomic energy watchdog agency of the United Nations will travel to Iran on Sunday for meetings about the country’s implementation of its nuclear agreement with major powers, the International Atomic Energy Agency said on Friday.

IAEA Director General Yukiya Amano expressed “concerns” to Iran last month over its repeated testing of a limit under that agreement on its stock of heavy water, a substance used as a moderator in some nuclear reactors.

Iran has since shipped its excess heavy water out of the country, but diplomats say it has yet to be delivered to a buyer. The United States and its allies say Tehran must do so to comply with the agreement.

“The visit is part of regular high-level contacts between the agency and Iran,” the IAEA, which is policing the restrictions the agreement places on Iran’s nuclear activities, said in a statement.

“In Tehran, the Director General will discuss Iran’s implementation of its nuclear-related commitments under the Joint Comprehensive Plan of Action (JCPOA),” it said, using the official name of the deal, which also lifted international sanctions against the Islamic Republic.

The row over heavy water raises questions about how U.S. President-elect Donald Trump will handle any such incidents after he takes office next month. He has said he would “police that contract so tough they (the Iranians) don’t have a chance”.

The visit also comes as Iran is complaining increasingly loudly that the United States has not held up its side of the bargain, a charge Washington denies.

Iranian President Hassan Rouhani ordered his country’s scientists on Tuesday to start developing systems for nuclear-powered marine vessels. He was responding to a decision by U.S. Congress to extend some sanctions against Tehran that would also make it easier to re-impose others.