BBC:

US and Iranian officials have arrived in Oman's capital Muscat to try to reach a new deal over Iran's controversial nuclear programme.

Donald Trump pulled the US out of a previous nuclear agreement between Iran and world powers in 2018, and reinstated economic sanctions, angering Iran.

Trump has warned of military action if the talks do not succeed.

It insists it is not trying to develop nuclear weapons, but many countries - as well as the global nuclear watchdog, the International Atomic Energy Agency (IAEA) - are not convinced.

Suspicions about Iran's intentions arose when the country was found to have secret nuclear facilities in 2002.

This broke an agreement called the Nuclear Non-Proliferation Treaty (NPT), which Iran and almost all other countries have signed.

The NPT lets countries use non-military nuclear technology - such as for medicine, agriculture and energy - but does not permit the development of nuclear weapons.

How advanced is Iran's nuclear programme?

Since the US pulled out of the existing nuclear deal - known as the Joint Comprehensive Plan of Action or JCPOA - in 2018, Iran has breached key commitments, in retaliation for the decision to reinstate sanctions.

It has installed thousands of advanced centrifuges (purification machines) to enrich uranium, something which was banned by the JCPOA.

Nuclear weapons require uranium which has been enriched to 90% purity. Under the JCPOA, Iran was only allowed to possess up to 300kg (600lb) of uranium enriched to 3.67% - sufficient for civilian nuclear power and research purposes but not nuclear bombs.

But by March 2025, the IAEA said Iran had about 275kg of uranium which it had enriched to 60% purity. That is enough to theoretically make about half a dozen weapons, should Iran further enrich the uranium.

US officials have said they believe Iran could turn that uranium into enough weapons-grade material for one bomb in as little as a week.

However, they have also said it would take Iran between a year to 18 months to build a nuclear weapon. Some experts say a "crude" device could be built in six months or less.

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