Iran's foreign minister has returned to Vienna for talks with world powers on the day set as a deadline for an agreement on its nuclear programme, according to BBC News.

Mohammad Javad Zarif is accompanied by the head of Iran's atomic energy agency and President Hassan Rouhani's brother.

All parties to the talks say they do not expect to reach a deal by the time the self-imposed deadline expires.

They stress that much has already been achieved, but a BBC correspondent says success remains far from certain.

The so-called P5+1 - the US, UK, France, China and Russia plus Germany - want Iran to scale back its sensitive nuclear activities to ensure that it cannot build a nuclear weapon.

Iran, which wants international sanctions lifted in exchange, insists that its nuclear work is peaceful.