Iran nuclear talks to go beyond 30 June deadline

The admission came as Iran's Foreign Minister Mohammed Javad Zarif prepared to fly back to Tehran from Vienna, Austria, where talks are taking place.

Observers said Mr Zarif probably needed to seek guidance over a stumbling block in negotiations - how much access Tehran will grant to nuclear monitors.

Six world powers and Iran are taking part in the talks.

The negotiations aim to see limits placed on Tehran's nuclear programme in exchange for international sanctions on Iran.

The so-called P5+1 - the US, UK, France, China and Russia plus Germany - want to limit Iran's sensitive nuclear activities to ensure that it could not build a nuclear weapon quickly.

The negotiators were a few days late agreeing a framework agreement, which was reached in early April, and it now appears likely that Tuesday's deadline for a comprehensive deal may also not be respected.

But the US official - who spoke on condition of anonymity - insisted there would be no long-term extension, tweeted the BBC's state department correspondent Barbara Plett.

Mr Zarif told Mr Kerry of his return to Tehran on Saturday, and he is expected to return on Monday, the US official said.

As well as the details of the proposed UN monitoring regime on Tehran's activities, disagreements are also reported to exist over the timing of sanctions relief.

US negotiators are being closely watched by critics both at home and in Israel - including Israeli Prime Minister Benjamin Netanyahu, whose country is understood to have the Middle East's only nuclear arsenal.

On Sunday he decried "this bad agreement, which is becoming worse by the day".