The UN's nuclear watchdog has refused to comment on the sudden departure of its chief inspector, days after Donald Trump took the United States out of the deal between Iran and world powers over its nuclear programme.

Today's dramatic resignation of Tero Varjoranta, who led the International Atomic Energy Agency (IAEA) comes at a sensitive time as Trump's decision has raised questions as to whether Tehran will continue to comply with the nuclear accord.

Tero Varjoranta, the chief inspector of the UN's nuclear watchdog the International Atomic Energy Agency (IAEA), above, has resigned (stock image)

The IAEA conducts inspections in Iran to verify compliance with the terms of the deal.

In its announcement of Mr Varjoranta's resignation, the agency did not give a reason for his departure. But, he has already been replaced in an acting capacity by a nuclear engineer, the head of the department's Iran team, the Vienna-based IAEA said.

'Mr Tero Varjoranta has resigned effective 11 May 2018,' an IAEA spokesman said, adding: 'The director general has appointed Mr Massimo Aparo, acting director, Office for Verification in Iran, as acting deputy director general and head of the Department of Safeguards, effective immediately.'

The dramatic resignation of Tero Varjoranta, who led the International Atomic Energy Agency (IAEA) comes at a sensitive time as Trump's decision has raised questions as to whether Tehran will continue to comply with the nuclear accord (stock image)

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

The accord signed by Iran and major powers in 2015 imposed strict limits on Iran's atomic activities to help ensure they are not put to developing nuclear bombs in exchange for the lifting of international sanctions against Tehran.

The IAEA is policing those restrictions and said on Wednesday, the day after Trump's announcement, that Iran was still implementing its commitments under the deal.

Tero Varjoranta is pictured here in 2015, as he arrives after his flight from Iran at Vienna Airport. IAEA Director General Yukiya Amano plans to appoint a permanent replacement as soon as possible

The UN watchdog has also repeatedly defended the landmark agreement, saying it is a gain for nuclear verification.

'The agency's safeguards activities will continue to be carried out in a highly professional manner,' the agency spokesman said.

Asked why Varjoranta had resigned, he replied: 'The agency cannot comment on personnel matters, which are confidential.'

IAEA Director General Yukiya Amano plans to appoint a permanent replacement as soon as possible, he added.