Iran has insisted that its nuclear programme is entirely peaceful. A landmark nuclear deal between Iran and world powers appears close to collapse. Here's an overview of the tensions and mistrust.

What is the essence of the deal?
Suspicions that Iran was using its nuclear energy programme as cover to develop a nuclear bomb prompted the UN, US and EU to impose sanctions with the aim of persuading the country to curb its sensitive nuclear activities. But Iran insisted its nuclear programme was peaceful,
It agreed to limit the enrichment of uranium, which is used to make reactor fuel but also nuclear weapons; redesign a heavy-water reactor being built, whose spent fuel would contain plutonium suitable for a bomb; and allow inspections by a global watchdog.

In return, relevant sanctions were lifted, allowing Iran to resume oil exports - the government's main source of revenue.

What prompted the latest crisis?
The administration of US President Donald Trump abandoned the deal in May 2018 and began reinstating sanctions. In November, those targeting Iran's oil and financial sectors took effect. They have triggered an economic meltdown in Iran and soaring inflation. Iran has now responded by scaling back its commitments under the deal.
Ill-feeling between the US and Iran has increased under the Trump administration. It suspended obligatory sales overseas of surplus enriched uranium and heavy water.

It also gave the five countries still party to the deal 60 days to protect Iranian oil sales from US sanctions. Otherwise Iran will suspend its restrictions on uranium enrichment and halt the redesign of its heavy-water reactor.

Global nuclear watchdog the IAEA says Iran has already increased production of enriched uranium - but it is not known by how much.