VIENNA - U.S. Secretary of State John Kerry said difficult issues on the Iranian nuclear talks still need to be resolved after meeting with Iran's foreign minster on Saturday.

"Still have difficult issues to resolve," Kerry said on his twitter.

Earlier on Saturday, Kerry met with Iranian Foreign Minister Mohammad Javad Zarif and European Union foreign policy chief Federica Mogherini.

As tough issues remain on the negotiation table, the deadline has been extended to next Monday, the third time extension.

But Iranian media reported the talks have no deadline.

On Friday, a source told Xinhua that big progress was made in certain issues of the nuclear talks, while works are still needed in a few remaining issues.

Iran and the five UN Security Council permanent members plus Germany have been negotiating over the past 18 moths to reach a long-term deal over Tehran's controversial atomic plan.(Xinhua)