Top US diplomat John Kerry meets his Russian counterpart Monday after trying to entice Syrian rebels into Geneva peace talks by vowing to exclude the Moscow-backed Damascus regime from any future transition government.

Kerry said he was "confident" the opposition would attend the upcoming talks in Switzerland, speaking after a meeting in Paris Sunday of the "Friends of Syria" group of mainly Western and Gulf countries opposed to Syrian President Bashar al-Assad.

The meetings come as Assad s forces appear to make gains on the ground amid deadly internal fighting in the rebellion pitting the once-dominant Islamists against resurgent fighters from groups more palatable to the West.

The US Secretary of State met National Coalition leader Ahmad Jarba, whose grouping includes the main opposition movement in exile, and was due to hold further talks with him on Monday.

"I am confident that he and others will be in Geneva. I am counting on both parties to come together," he said.

Jarba was equally upbeat, saying a "milestone" had been passed in diplomatic efforts to end the conflict, which is believed to have killed at least 130,000 people in nearly three years.

In a statement, the 11-nation Friends of Syria group said that once a transitional government is established "Assad and his close associates with blood on their hands will have no role in Syria".

The so-called "Geneva II" talks due to start on January 22 have been organised in an attempt to revive the idea of moving to a transitional government including figures from the current regime and the opposition.

Will Iran attend?

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Whether that could involve Assad himself is an issue that has generally been fudged in the past and may have the potential to capsize the negotiations: Assad s aides have repeatedly said they have no intention of coming to Switzerland to hand over power.

That issue is likely to feature prominently in Kerry s discussions Monday with Russian Foreign Minister Sergei Lavrov and UN-Arab League envoy Lakhdar Brahimi.

Another key point in their talks will be whether Iran, which has been instrumental in propping up Assad, should have any role in peace talks further down the line.

"I have said many times, publicly and privately, I would welcome any initiative Iran wishes to take, if they do, to try to provide a resolution to the crisis of Syria," Kerry said.

"The first thing they can do is accept the Geneva communique," he said, in reference to a first round of talks held in June 2012 which called for a transitional government recognising the opposition.

Iran announced Sunday through an official television network that its foreign minister, Mohammad Javad Zarif, would travel to Damascus in the next few days.

"If we receive an invitation without any preconditions, we will participate in the  Geneva 2  peace conference, but we won t act in order to receive an invitation," Zarif said in Beirut.

Kerry would not be drawn on what would happen if Assad pulled out of the talks.

"With respect to the Assad regime we have been told from day one they allegedly are prepared to negotiate," he said.

Rebel infighting kills 700

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Jarba has previously called for Assad to stop using heavy weapons, lift sieges on a number of opposition-held areas and allow the opening of humanitarian corridors as a show of good faith ahead of any talks.

There has been no sign of progress on those issues but, with little prospect of securing a military victory, the opposition has few alternatives to negotiations.

"This is going to put the Assad regime on the spot if everybody turns up at those peace talks," British Foreign Secretary William Hague told Sky News from Paris on Sunday.

However, the balance of power appears to have tipped in Assad s favour over the last week as deadly clashes have erupted between the mainstream opposition and an Al Qaeda-linked group, the Islamic State of Iraq and the Levant (ISIL), with which they were previously allied.

According to NGOs monitoring the conflict, at least 700 people have been killed since the fighting started January 3.