John Kerry has arrived in Austria's capital Vienna for talks with foreign ministers from the six powers negotiating with Iran on its nuclear programme, with one week left to bridge major differences and strike a historic nuclear deal.

The US secretary of state arrived on Sunday after clinching a deal in Kabul with Afghanistan's presidential candidates to hold a UN-monitored audit of the country's disputed election.

The US, UK, France, Germany, Russia and China are involved in talks to reach a long-term deal to end the decade-old nuclear dispute with Iran by July 20.

Officials on both sides say that the talks, which on July 3 entered their sixth and final round, have progressed, with Iran's chief negotiator saying on Saturday that a draft accord was 60-65 percent complete.

One key notable issue that remains is uranium enrichment, a process that can produce fuel or weapons-grade material.

Iran wants to increase its capability to fuel its only functioning nuclear power station, and future planned facilities. The six powers want a sharp reduction. 

Kerry "will gauge the extent of Iran's willingness to commit to credible and verifiable steps that would back up its public statements about the peaceful nature of its nuclear programme," the US state department said.

He will "assess Iran's willingness to make a set of critical choices at the negotiating table" and then "make recommendations" to the US president, Barack Obama.

Many diplomats and analysts believe an extension may be needed in view of the wide gaps in negotiating positions.

Among his meetings on Sunday, Kerry will meet with his German counterpart Frank-Walter Steinmeier, who has said he wants to discuss new accusations of US spying on Berlin.

Source: Agencies