A top American nuclear expert has handed his resignation to President Donald Trump after Washington’s unilateral withdrawal from the Iran nuclear agreement this week.

Richard Johnson, the assistant coordinator for Iran nuclear issues at the Office of Nuclear Implementation, stepped down without giving an explanation.

Johnson had worked as a non-proliferation officer in the US State Department since 2006.

He had been involved in Washington's weeks-long negotiations with the United Kingdom, France and Germany, during which the European governments, strived to salvage the agreement.

In an email circulated to his colleagues and staff, Johnson has described the Iran deal “an extraordinary achievement.”

“I am proud to have played a small part in this work, particularly the extraordinary achievement of implementing the [deal] with Iran,” he wrote.

The landmark deal, officially known as the Joint Comprehensive Plan of Action (JCPOA), signed between Iran and the P5+1 group of countries — the US, UK, France, China, Russia and Germany — in 2015.

Under the agreement, Tehran undertook to put limits on its nuclear program in exchange for the removal of nuclear-related sanctions imposed against Iran.

Trump, who has consistently described the deal as “the worst and most one-sided transaction Washington has ever entered into,” eventually announced on Tuesday that he pulls out the US from the accord. He also announced a re-imposition and strengthening of sanctions against Iran.

Laura Kennedy, a former ambassador and a board member of the Center for Arms Control and Non-Proliferation, wrote in a Twitter message that “Johnson is among the best and the brightest.”

“His departure from the [US government] is part of the huge, worldwide collateral damage that is being wrought by Trump’s violation of the Iran deal,” she wrote.

A US official also described Johnson’s resignation a “big loss” for the government.

The report of Johnson’s resignation came a day after the United Nations’ top nuclear inspector unexpectedly stepped down without giving any explanation.

The International Atomic Energy Agency (IAEA) announced late Friday that Tero Varjoranta had stepped down.

Varjoranta had been a deputy director general of the IAEA and head of its Department of Safeguards, which verifies countries’ compliance with the nuclear Non-Proliferation Treaty, since October 2013.

Following Trump's unlawful withdrawal from the deal, the IAEA said that Iran was still implementing its commitments under the deal, defending it as “a gain for nuclear verification.”