GENEVA: US Secretary of State John Kerry cautioned Friday there was no deal yet with Iran over its nuclear programme, as he and Western counterparts joined Geneva talks in a bid to clinch a landmark agreement.

Kerry cut short a Middle East tour to join the negotiations. The foreign ministers of Britain, France and Germany also flew in to take part, lifting the status of the talks between six world powers and Iran.

“There is not an agreement at this point,” Kerry told reporters on arrival, adding: “There are still some very important issues on the table that are unresolved.”

”It is important for those to be properly, thoroughly addressed,” he said, adding though that world powers were “working hard” to reach an agreement.

Iranian Foreign Minister Mohammad Javad Zarif has said he believes a preliminary deal could be struck during this round in Geneva.

The talks, which started Thursday, were meant to end late Friday but it now looked likely they would be extended into Saturday.

If some sort of agreement is reached, it would be a breakthrough after a decade of negotiations between Iran and the P5+1 group comprising the United States, Britain, France, Germany, Russia and China.

The outlines of a proposal Iran made in the previous round last month have been kept confidential by both sides in a bid to advance positions.

The hoped-for agreement, seen as a first step ahead of further talks on a final deal, could see Tehran freeze its nuclear efforts for as long as six months in exchange for some relief from the sanctions that have battered its economy.

Reports say the proposed deal could see Tehran stop enriching uranium to 20 percent, which is just a few technical steps from weapons grade, reduce existing stockpiles and agree not to activate its plutonium reactor at Arak.

Global powers would in exchange take limited and “reversible” measures to ease sanctions, such as unfreezing some Iranian funds in foreign accounts.

Negotiators would then have time to work out a more comprehensive deal that Tehran has said it hopes could be in place within a year.

French Foreign Minister Laurent Fabius was also cautious in Geneva. “There has been progress but nothing has been agreed yet,” he said.

British Prime Minister David Cameron, who sent Foreign Secretary William Hague to Geneva, said in a Twitter message Friday that the talks “offer an opportunity to make real progress” on the Iran nuclear question.

Iran's lead negotiator, Deputy Foreign Minister Abbas Araqchi, said progress was being made but warned that talks could drag on.“We cannot say whether the negotiations will bear fruit tonight (Friday) or not, despite the good progress,” he was quoted as saying by the ISNA news agency.

“There is a need to reach an agreement, the negotiations could go on to midnight or continue Saturday, or be postponed to the next round.”

'Deal of the century'

But the putative deal already came fire under from Israel, which has staunchly opposed any easing of sanctions against Iran, and demanded a halt to all enrichment there.

Israeli Prime Minister Benjamin Netanyahu, who hosted Kerry on a brief stopover before Geneva, urged world powers to back away from the talked-about agreement.

“Iran got the deal of the century and the international community got a bad deal, this is a very bad deal. Israel utterly rejects it,” Netanyahu told reporters.

Washington rejected Israeli criticism as “premature”, with White House spokesman Josh Earnest saying aboard Air Force One: “There is no deal. Any critique of the deal is premature.”

Iran has repeatedly denied its nuclear programme is for anything other than generating electricity and for medical purposes.

But the world powers in the talks, which include the five permanent members of the UN Security Council, which has issued multiple resolutions against Iran's uranium enrichment, suspect Tehran's programme was aimed at developing nuclear weapons.

The head of the UN atomic watchdog, Yukiya Amano, will travel to Tehran on Monday to discuss “technical issues” related to monitoring of Iran's nuclear programme, the agency said in a statement.

The two levels of talks with Iran over its atomic activities were given new momentum by the June election of Iranian President Hassan Rouhani, seen as a relative moderate who has the backing of the man who has the final say: supreme leader Ali Khamenei.

Iran is anxious for relief from crippling US and EU economic sanctions that have cut oil revenues by more than half, caused the value of the rial to plunge and pushed inflation above 40 per cent.

The West is also keen to seize a rare opportunity to build bridges with Iran after decades of hostility, opening the door to engaging with Tehran on other issues such as the conflict in Syria, where Iran has backed President Bashar al-Assad against insurgents.