U.S. Democratic presidential frontrunner Hillary Clinton said Monday that putting American combat troops on the ground in Syria or Iraq to fight the Islamic State group was a "non-starter."

"Well, at this point I cannot conceive of any circumstances where I would agree to do that because I think the best way to defeat ISIS is, as I've said, from the air which we lead, on the ground, which we enable, empower, train, equip and in cyberspace where, don't forget, they are a formidable adversary online," Clinton said.

Iran said Monday that Saudi Arabia's plan to host a meeting of armed and political opponents of Syrian President Bashar Assad would breach declarations made during recent international peace talks.

Tehran and Riyadh are on opposite sides of the conflict in Syria but both have taken part in two major meetings in Vienna aimed at finding a political solution.

Syria on Monday denied ever using chemical weapons in its four-year-old civil war, telling a global watchdog it was cooperating fully with the destruction of its toxic stockpile.

Damascus's rebuttal comes amid growing accusations it is not being transparent with the world's chemical watchdog and U.N. efforts are stepped up to track down the perpetrators of deadly chlorine gas attacks in the war-torn country last year.

Working with the Syrian army to fight the Islamic State group is not on the cards until President Bashar al-Assad has been removed from power, French Foreign Minister Laurent Fabius said Monday.

France's top diplomat said it was "obvious" Assad could not work alongside moderate rebels in Syria. 

The Kremlin said Friday that Western powers were not ready to form a coalition with Russia to fight Islamic State jihadists in Syria, after talks between President Vladimir Putin and French leader Francois Hollande.

"At the moment, unfortunately, our partners are not ready to work within the format of single coalition," Putin's spokesman Dmitry Peskov told reporters.

French Foreign Minister Laurent Fabius said Friday he could envisage Syrian regime troops taking part in the fight to crush the Islamic State group.

However, Fabius later clarified his comments, saying he meant that Syrian government troops could take part in the fight against IS only after a change of regime.

In the Islamic State group's Syrian stronghold of Raqa, sirens ring out whenever a warplane approaches as jihadists flee their posts and vehicles to hide, activists say.

A U.S.-led coalition and Russia have stepped up air strikes on the jihadists' de facto Syrian capital since IS claimed to have downed a Russian passenger plane over Egypt's Sinai in October and the deadly jihadist attacks in Paris two weeks later.

Prime Minister Tammam Salam chaired on Thursday a meeting of the committee tasked with addressing the case of Syrian refugees in Lebanon, reported al-Joumhouria newspaper on Friday.

Sources from talks told the daily that Lebanon has incurred in the past year 16 billion dollars in losses as a result of the crisis.

U.S. soldiers are in Kobane, the town in northern Syria nearly destroyed in fierce fighting with the Islamic State group, to train Kurdish forces to battle the jihadists, Kurdish sources said Thursday.

Mustapha Abdi, an activist in the town on the Turkish border, told AFP the American instructors had arrived "in recent hours".

Germany plans to send Tornado reconnaissance jets to support the fight against the Islamic State group in Syria, the defense spokesman of Chancellor Angela Merkel's conservatives said Thursday.

A day after Merkel pledged in Paris to "very soon" decide how to help battle the IS group in Syria, she met cabinet ministers in charge of security ahead of afternoon meetings of the major parties' parliamentary groups.