Iran's foreign minister has called for an extension to the looming deadline to agree an historic nuclear deal with world powers after talks with US Secretary of State John Kerry.

Kerry met with Iranian Foreign Minister Mohammad Javad Zarif after two days of intense talks in Vienna ahead of a 20 July deadline and the prospect of an extension to the nuclear talks.

"As we stand now, we have made enough headway to be able to tell our political bosses that this is a process worth continuing," said Zarif.

"This is my recommendation. I am sure Secretary Kerry will make the same recommendation."

In a press conference, Kerry said that there had been some "tangible progress" in talks despite "very real gaps on other key issues".

"I am returning today to Washington to discuss with President Obama and leaders in Congress over the coming days about the prospects for a comprehensive agreement, as well as the path forward if we do not achieve one by 20 July," he said.

Kerry said that talks with Obama would consider "the question of whether or not more time is warranted, based on the progress we have made and how things are going.

"With respect to the issue of 20 July, yes, that is still on the table. We are still working and we are going to continue to work."

Kerry would not reveal the fine details of the negotiations with Zarif, but said that the 19,000 centrifuges that Iran held for uranium enrichment was too much.

The US, the UK, France, Germany, Russia and China, otherwise known as the P5+1, are involved in the Vienna negotiations and all parties are looking for Iran to reduce its capacity in order to hinder the chances of a nuclear weapon being built without international inspection.

Iran has denied that it is seeking to build a large-scale nuclear bomb and is willing to negotiate a potential freeze to its nuclear capacity in return for the removal of crippling economic sanctions.