Vienna: Three European powers and the US have confirmed that their foreign ministers will join the upcoming Iran nuclear talks in an effort to advance the stalled negotiations.

But the Itar-Tass news agency reported Friday that their Russian counterpart, Sergey Lavrov, isn’t coming. That could lessen chances of success, because Moscow often acts as a bridge between Washington and Tehran at the talks.

Foreign ministers from Britain, France and Germany are expected in Vienna by Sunday. US Secretary of State John Kerry is scheduled to arrive Saturday.

The four hope to lessen significant disagreements hindering attempts to agree on long-term curbs on Iran’s nuclear activities in exchange for ending sanctions on Tehran.

If that fails, the two sides could extend their negotiations past a July 20 target date for a deal.

“If Kerry is to help resolve the deadlock, instead of trying to narrow the unbridgeable gap over Iran’s enrichment capacity, he should try to broaden the options,” Ali Vaez, an Istanbul-based senior analyst for the International Crisis Group, said in a written response to questions. “Both sides know that there is real cost to declaring failure.”

Following 10 days of talks, diplomats haven’t shown they are any closer to solving their most intractable disagreements. Iran says it wants to expand uranium-enrichment work that can be used to fuel nuclear reactors. The US and its allies want the nation to reduce enrichment activities, which can also yield material for nuclear weapons.

European Union foreign-policy chief Catherine Ashton, who together with Iran’s Foreign Minister Mohammad Javad Zarif convened the talks, invited top diplomats to participate earlier in the week. The offices of French Foreign Minister Laurent Fabius, German Foreign Minister Frank-Walter Steinmeier and UK Foreign Secretary William Hague also confirmed their participation.