GENEVA (Reuters) - Syria peace talks being hosted by the U.N. in Geneva have descended into bureaucratic wrangling since resuming this year, and even an optimist would struggle to conclude they are inching toward a settlement between the warring sides.

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But behind the scenes, the rumor mill has focused on one question that seems designed to stop their mediator, Staffan de Mistura, from making any sort of progress: is he about to leave?

While western diplomats are keen for him to stay on, Russian media outlets have cited unnamed sources saying that de Mistura’s mandate expires on Friday and he will formally retire in April, before a new round of talks in May.

Other Russian reports say the veteran diplomat, who turned 70 in January, will stay on for a further six months.

Russia, the chief ally of Syrian President Bashar al-Assad, is widely seen as holding the balance of power in the talks, as well as on the battlefield. Last year Foreign Minister Sergei Lavrov accused de Mistura of “sabotaging” peace talks, before apparently approving of him again.

Diplomats say they do not know what Antonio Guterres, who took over as U.N. Secretary-General on Jan. 1, has in mind, but say the U.N. coordinator in Lebanon, Sigrid Kaag, is being floated as a potential successor.

De Mistura has declined to say if he is going or staying, since becoming a lame duck could undermine his ability to cajole the Syrian negotiators toward a deal.

This month he said that upon the arrival a new U.N. Secretary-General, the top U.N. officials stayed for another three months maximum - or longer if their boss wanted them to.

“But there is also one important person who indicates to me how long I should be staying, that is my wife. So we have quite a negotiation on that,” de Mistura said.

Western diplomats say de Mistura, who took on the role in July 2014 after his predecessors Lakhdar Brahimi and Kofi Annan failed to make peace, had built a strong team and the talks would benefit from keeping him on.

“We’ve heard this kind of rumor before. Obviously there’s a lot of it this round, but nothing to confirm or deny either way to suggest if he’s actually going,” one diplomat said.

“There’s been some pressure put by way of media statements and leaks from the Russian side. That’s the main source that I’ve seen of pressure. What the Russians intend to do with that is not something I completely comprehend.”

Another Western diplomat said the Syrian opposition was “obsessed” with the issue.

But opposition negotiator Basma Kodmani told Reuters there was no preference about the mediator’s identity, as long as any newcomer did not waste months getting to grips with the issues and the personalities involved.

“That person needs to be fully aware of everything, not put the clock back and say: ‘Oh, the regime maybe might...’ No it might not,” she said.

“Six years later you do not start all over again. That is our biggest concern.”