Iran has purposely avoided negotiations and used the time since the last meeting in June to further expand activity that the West suspects is aimed at enabling it to build a nuclear bomb, something that Israel has suggested it will prevent by force if diplomacy fails.

The two day negotiations in Kazakhstan were precipitated by unsuccessful talks in Istanbul, Baghdad, and Moscow.

A U.S. official said on Monday that offer from the P5+1 to Iran  (an updated version of one rejected by Iran last year) would take into account its recent nuclear advances, but also take "some steps in the sanctions arena".

This offer would address some of Iran's concerns but not meet its full demands. Iran is currently demanding that all sanctions be lifted.

A Western official later said the powers had formally presented the offer during Tuesday's talks but gave no details.

In Almaty, a source close to the Iranian negotiators told reporters: "Depending on what proposal we receive from the other side we will present our own proposal of the same weight. The continuation of talks depends on how this exchange of proposals goes forward."

Iranian media also said the talks would continue, without saying whether the Iranian proposal had been presented.

U.S. Secretary of State John Kerry said in Berlin that he hoped Iran "will make its choice to move down the path of a diplomatic solution".

A Western official who declined to be named said the talks had been "useful" and confirmed they would continue on Wednesday as scheduled.

But with the Islamic Republic's political elite preoccupied with worsening infighting before a presidential election in June, few believe the meeting will yield a quick breakthrough.

"It is clear that nobody expects to come from Almaty with a fully done deal," said a spokesman for the European Union's foreign policy chief Catherine Ashton, who oversees contacts with Iran on behalf of world powers.​