Yukiya Amano, the head of the global nuclear watchdog the IAEA, has visited Iran's Parchin military site, BBC News reports.

Western intelligence agencies have long suspected that nuclear arms research took place at Parchin. Iran has denied this and insists its nuclear programme is purely peaceful.

Amano arrived in Iran on Sunday, September 20. His visit is seen by Western diplomats as key to the implementation of a recent deal with world powers over Iran's nuclear programme.

International inspectors have previously only had limited access to the complex.

Under the new deal inspectors from the IAEA will continuously monitor Iran's declared nuclear sites and verify that no fissile material is moved covertly to a secret location for a bomb.

Iran has also agreed to allow inspectors to access any site they deem suspicious.

Concerns about Parchin's possible role in Iran's nuclear programme first emerged in 2004.

In a 2006 report, the IAEA said its inspectors "did not observe any unusual activities in the buildings visited, and the results of the analysis of environmental samples did not indicate the presence of nuclear material".

But concerns have persisted. In late 2011, the IAEA said it observed extensive landscaping, demolition and new construction at the site.

In February 2012, inspectors were turned away from the site.

The deal agreed in July entailed Iran agreeing to limit its sensitive nuclear activities in return for an end to crippling international sanctions. The U.S. says the deal will prevent Iran from acquiring a nuclear weapon. U.S. Republicans had opposed the deal but an attempt to derail it was been blocked by Democrats in the Senate earlier this month.