(Reuters) - Iranian Foreign Minister Mohammad Javad Zarif began talks with U.S. Secretary of State John Kerry and European Union envoy Catherine Ashton in Oman on Sunday to try to advance efforts to end a standoff over Tehran's nuclear programme, a witness said.

The discussions, aimed at curbing Iran’s sensitive uranium enrichment work in return for a gradual lifting of sanctions, are taking place just two weeks before a self-imposed Nov. 24 deadline for reaching a comprehensive deal. Iranian official media also reported the start of the Muscat talks.

The West suspects Iran has covertly sought to develop the means to build nuclear weapons, but the Islamic Republic says its programme is entirely for peaceful purposes.

The thorniest unresolved issues are Iran's overall uranium enrichment capacity, the length of any long-term agreement and the pace at which international sanctions would be phased out, according to Western diplomats involved in the negotiations.

As Kerry arrived in Oman early on Sunday, a senior U.S. official said the three-way talks would be "an important meeting," with the focus on making progress in order to meet the deadline.

U.S. officials say major gaps still remain in the two sides’ negotiating position. Kerry said last week that the United States and its partners were not contemplating an extension of the Nov. 24 deadline, although he held out the possibility that negotiations could go beyond that date if major issues were agreed and there were only technical details to wrap up.

Speaking to Iranian state television on his arrival in Muscat on Saturday night, Zarif reiterated that sanctions imposed on Iran had brought 'no result' for the West.

"We need to reach a solution based on mutual respect and cooperation. If the West is interested in reaching such a solution, there is possibility to find a solution and to reach an understanding before November 24," he said.

A senior Iranian official close to the talks told Reuters that the participants would discuss "the gaps that are still huge, Iran's (uranium) enrichment capacity and timeframe of lifting sanctions."

(Reporting by Warren Strobel, Parisa Hafezi and Fatma Al-Araimi, Editing by William Maclean)