Six world powers including the US, Russia and the UK have been negotiating with Iran about scaling down its nuclear activities.

It is hoped that Iran will agree to do so imminently in return for the lifting of some economic sanctions.

The latest deal deadline on Monday night passed with no announcement.

But Iran`s Students News Agency (Isna) reported early on Tuesday that Iran`s Foreign Minister Mohammad Javad Zarif and EU foreign policy chief Federica Mogherini would read a joint statement in Vienna "in a few hours".

Meanwhile another Iranian news agency, the semi-official Fars, said Iran would meet the six countries` representatives at 08:00 GMT on Tuesday for " a comprehensive meeting".

There were contrasting assessments about the possibility of an agreement as hopes of a breakthrough rose and then receded throughout Monday.

Diplomats familiar with the talks told the Associated Press news agency that an announcement of a deal could be approaching because of the need of top officials involved in the negotiations to leave Austria`s capital on Tuesday morning.

However, the White House cautioned that significant issues remained unresolved and that Iran still had some tough decisions to make. even though "genuine progress" had been made and the US negotiating team was prepared to stay in Austria as long as there was a chance of success in the talks.

"They`re not going to sign an agreement until all of our concerns have been addressed.

And as long as they continue to make progress in doing that, then the talks will, will continue," White House spokesman Josh Earnest said.

Iranian FM Zarif said that while a breakthrough was imminent, there would be no deal before Tuesday.

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 crippling international sanctions lifted, has always insisted that its nuclear work is peaceful.

`Tired and overworked`

The BBC`s James Robbins in Vienna says the clocks are running exceedingly slowly in the negotiations.

All the key ministers seem close to exhaustion, sending out conflicting signals on the timing of an agreement, our correspondent adds.

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