US Secretary of State John Kerry has arrived in the Austrian capital Vienna for talks on Iran's nuclear energy program.

Negotiations between Tehran and the P5+1 group of countries have been underway in Vienna at deputy and expert levels.

Kerry arrived in Vienna late on Friday.Iranian Foreign Minister Mohammad Javad Zarif will be joining the talks on Saturday.

Iranian Deputy Foreign Minister Abbas Araqchi said the talks are making progress, but at a slow pace.

"Some major problems exist which are still blocking the work... but in other areas we have made good progress," he said on Friday.

"Overall, the work is moving ahead slowly and with difficulty," he added.

He said the negotiating sides still remain divided over a number of issues while working on a draft of a possible final nuclear deal.

Araqchi noted that the divisions even exist among the P5+1 group – the US, Britain, France, China, Russia and Germany – over sensitive issues.

An unnamed Western diplomat involved in the talks told AFP that several key issues, including future UN inspections and the timing of sanctions removal, remain "extremely problematic".

"The most difficult issues need to be resolved in the coming days: transparency, inspections, PMD (possible military dimensions of Iran's nuclear energy program), sanctions... On the major issues there is major disagreement," the diplomat said.

The two sides have until the end of June to reach a comprehensive final agreement.

On Wednesday, Kerry, who is still walking on crutches after breaking his leg in a cycling accident in France late last month, said there will be no nuclear deal with Iran if outstanding issues aren't resolved in the talks.

"It may be that the Iranians will not fill out the full measure of what was agreed on in Lausanne, in which case there will not be an agreement," Kerry told reporters in Washington, DC.

He was referring to the outlines of a landmark deal agreed on April 2 in Switzerland between Iran and the P5+1 group.

Kerry said whether or not Iran and the P5+1 succeed in reaching an agreement "will be determined in the last days, by whether or not the outstanding issues that we've been very clear about are in fact addressed.”

GJH/GJH