Top level diplomats of the US and Germany have said that there were still serious gaps in talks over Iran's nuclear programme, despite signs of some progress two days before a deadline to reach a deal between Iran and major powers.

Six major powers - the US, UK, France, Germany, Russia and China -  began a final round of talks  with Iran in Vienna on Tuesday, looking to seal a deal under which the country would curb its nuclear work in exchange for a lifting of economically crippling sanctions.

"We are working hard ... We hope we are making careful progress, but we have big gaps, we still have some serious gaps, which we're working to close," Kerry, US secretary of state, said on Saturday in the Austrian capital after meeting Frank-Walter Steinmeier, the German foreign minister.

On his part, Steinmeier said that the outcome of Iran's nuclear talks with six world powers was "completely open".

"I think I can say that we have never been so close to a deal and that the atmosphere of the negotiations is very constructive, but we should be aware that there are still big gaps on certain issues," he said as he joined the talks.

"If Iran is ready to take this opportunity then movement is possible ... Whether we can get a result is right now completely open."

Philip Hammond, British foreign secretary, said on Friday: "We have to get more flexibility from the Iranians ... . In return we are prepared to show some flexibility on our side. But time is short. We are up against a deadline [Monday] here."

Western powers suspect Iran has aimed to covertly acquire a nuclear bomb capability from its enrichment of uranium.

Iran says the programme is for producing civilian energy only.

Iranian officials have refused to reduce the volume of uranium they are capable of enriching, a stand Western officials say is unacceptable as this would potentially allow Iran to amass enough fissile material for an atomic bomb in little time.

Another sticking point is the pace and sequencing of sanctions relief. Iran wants them terminated swiftly, not suspended and gradually scrapped, depending on the degree of Iranian compliance with the deal terms, as the West wants.

Another dispute is over the deal's duration: Western countries want it to be up to 20 years but Iran wants this much shorter.

An interim accord struck on November 24 last year yielded steps by Iran to curb some sensitive nuclear activity, such as higher-grade enrichment, in return for some sanctions relief.

Source: Agencies