By Louis Charbonneau
PRAGUE (Reuters) - U.S. Secretary of State John Kerry will travel to Vienna this weekend for a meeting of the six powers and Iran as they try to reach a deal that would end sanctions in return for curbs on Tehran's nuclear program, the State Department said on Thursday.
"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 in a statement.
Iran and the powers - the United States, France, Germany, Britain, Russia and China - aim to reach a long-term deal to end the decade-old nuclear standoff by a July 20 deadline. Many diplomats and analysts believe an extension may be needed in view of the wide gaps in negotiating positions.Kerry has scheduled bilateral meetings with fellow ministers on Sunday, a senior Western official told Reuters.
A French diplomatic source said Foreign Minister Laurent Fabius would arrive on Sunday for the talks.
Earlier this week, diplomats close to the talks said the ministers may end up negotiating the terms of an extension while in Vienna.
The talks resumed last week with still no sign of substantive progress on the main sticking points, which include uranium enrichment, the length of any agreement and the speed at which sanctions would be lifted.
The Western official said it was unlikely the ministers were going to Vienna to sign an agreement, given the significant differences between the parties on the acceptable scope of Tehran's future uranium enrichment program.
Another Western diplomat said: "Things have been moving but there are still wide gaps and they need to be narrowed down. This is what needs to happen in the next days."
French officials have said that Fabius was reluctant to go to Vienna until closer to the July 20 deadline unless there was a clear sign of a breakthrough.
"Major differences remain and it was felt that mid-July was too soon as there is still a lot of work, but since the start of the week there has been a desire from some powers to hold a ministerial meeting," said one official.
Russia's Foreign Ministry said on Thursday the ongoing round of talks was tough but expressed hope that it was possible to reach a deal by the deadline.
"The discussions are extremely difficult, but on the face of it there is some progress," Russian spokesman Alexander Lukashevich told a news conference.
"We hope to work out a final text of the agreement - despite all the difficulties - by the July 20 deadline."
Iranian Supreme Leader Ayatollah Ali Khamenei said on Tuesday that Iran would need to boost its uranium enrichment capacity in the long term, underlining a gap in positions but possibly signaling some flexibility in the short term.
(Additional reporting by Justyna Pawlak in Brussels, Gabriela Baczynska in Moscow, Fredrik Dahl in Vienna and John Irish in Paris; Editing by Robin Pomeroy, Peter Cooney and Ken Wills)