VIENNA – The fate of the diplomatic effort by world powers to reach agreement with Iran over its nuclear program is in jeopardy ahead of a key deadline, as US Secretary of State John Kerry travels to Vienna this weekend to test the Islamic Republic’s seriousness at the negotiating table.In the Austrian capital – where delegations from the US, UK, France, Russia, China, Germany and Iran have set up camp for three weeks in an attempt to forge a comprehensive resolution to the nuclear crisis – Kerry will meet with the EU’s Catherine Ashton and foreign ministers participating in the negotiations, and with Iranian Foreign Minister Mohammad Javad Zarif himself in a rare bilateral meeting.The one-on-one exchange will be the second time Kerry and Zarif have met face-to-face since direct relations between the US and Iran resumed last September at the United Nations General Assembly in New York.“The secretary 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 program,” State Department deputy spokeswoman Marie Harf said on Thursday night.“The secretary will see if progress can be made on the issues where significant gaps remain and assess Iran’s willingness to make a set of critical choices at the negotiating table.”Kerry will then make recommendations to US President Barack Obama about “next steps in the negotiations,” Harf said.The US has publicly questioned the wisdom of extending the talks, set to expire on July 20. Privately, however, members of the negotiating team have admitted that an extension might be necessary, with the alternative unclear, or unappetizing.The Iranian Foreign Ministry has for months expressed a willingness to extend the talks. An interim deal reached between the parties last November in Geneva allows for such an extension, of up to six months from July 20, should all parties agree.Iran has taken preparatory action to start up a uranium conversion plant it needs to fulfill an interim nuclear agreement reached with six world powers last year before the accord expires this month, according to diplomatic sources.The launch of the facility would show Tehran’s commitment to the November 24 deal as it holds talks with the US, Russia, France, Germany, Britain and China on a long-term settlement of the dispute over its atomic aims.The major powers want Iran to significantly scale back its nuclear program to deny it any capability to quickly produce atomic bombs. Iran wants sanctions lifted as soon as possible.In view of wide differences in positions, some diplomats and experts believe the negotiations – and the preliminary agreement – may need to be extended.But a spokesman for European Union foreign policy chief Catherine Ashton, who leads the negotiations on behalf of the powers, said they remained determined to try to get a comprehensive agreement by a self-imposed July 20 deadline.“We see seriousness also on the other side,” Michael Mann told reporters. “It is difficult, there are still significant gaps, but that is not a surprise. We are talking about very complicated technical issues and also very complicated political issues. So we are working hard to try to narrow those gaps.”Under the initial accord that runs for six months until July 20, Iran is supposed to convert a large amount of low-enriched uranium gas into an oxide form that would be less suitable for processing into nuclear bomb material. It was one of the terms of the deal that won Tehran easing of sanctions.To be able to do that, it has been building a facility near the central city of Isfahan for turning the gas into powder.After months of delays, the UN’s International Atomic Energy Agency in May said the plant’s commissioning had begun, but it was still not operating.Since then, however, the sources said practical steps had been taken indicating the work could start soon, if it had not already.Reuters contributed to this report.