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American President Barack Obama has agreed to a Russian military deployment in Syria on the condition that the role of Russian forces be limited to fighting the Islamic State group [IS], said a senior American official on Tuesday.

The American official who wished to remain anonymous told journalists that the meeting between Obama and Russian President Vladimir Putin at the UN General Assembly was "focused" and included discussions of Syria and Ukraine.

The American official said that the United States does not view Russian forces in Syria as being "necessarily destructive" to a positive resolution, but the America administration's view is dependent on how Russia acts in the coming period.

The US does not oppose Russian forces if their only mission was to fight IS, however if the mission "was to bolster Assad in his battle against his people, that would be negative," said the source.

These statements come after the US had expressed its surprise at the Russian military activities in Syria, which run contrary to reports of secret US-Russian deals.

The American official stressed that disagreements continue on the role that will be played by Assad, as the Russians believe him to a bulwark against extremists, while Americans believe that he is "fanning the flames of sectarian conflict in Syria".

"I think the Russians certainly realise the importance of providing a political solution in Syria and there is a course to achieving a political solution," said the official who added: "We have disagreements about the result of this course," in reference to the fate of Bashar al-Assad.

The meeting between Obama and Putin on the sidelines of the UN General Assembly last 90 minutes and was described by American officials as "constructive" and included a mutual desire to find a solution "without attempts to score points".

According to the source, Obama and Putin also agreed to US-Russian military coordination to avoid any friction in Syria or a regional conflict between the two countries.

Obama also urged on Tuesday that Assad must go if the Islamic State group is to be defeated.

"In Syria... defeating ISIL requires, I believe, a new leader," Obama told the gathering, held on the sidelines of the UN General Assembly.

Stop barrel bombs 
 
However, hinting at a possible compromise, US Secretary of State John Kerry said Washington could cooperate on Syria if Russia and Iran persuade Assad to stop using barrel bombs against civilians.

"They are both in a position, in exchange perhaps for something that we might do, they might decide to keep Assad from dropping barrel bombs," Kerry said in an interview with MSNBC.

Western diplomats maintain that Assad has killed more civilians by using barrel bombs dropped from helicopters than IS in its brutal advance in Syria. 

Taking stock 
 
Obama vowed to crush IS in his UN speech a year ago and called on countries to join the United States in the campaign.

Taking stock one year on, Obama said IS had lost a third of the "populated areas" it controlled in Iraq and had been "cut off" from almost all of Turkey's border region.

But he added that military action alone would not succeed unless efforts were made to address the conditions that allow Islamic radicalism to thrive.

Iraqi Prime Minister Haider al-Abadi called for international aid to equip his troops fighting the extremists, who triggered alarm after seizing the city of Mosul in June last year.

Since then, IS fighters have captured territory in Syria and Iraq and gained a foothold in Libya, Yemen and elsewhere in the Middle East, with alliances as far afield as Nigeria, with Boko Haram.