Syrian leader Bashar al-Assad must leave office or face being turfed out by force, Saudi foreign minister Adel Al-Jubeir says.

Mr Jubeir rejected Russian calls for an international coalition to help Mr Assad fight the Islamic State group, and said the Syrian leader's best option would be to step down.

"There is no future for Assad in Syria, with all due respect to the Russians or anyone else," Mr Jubeir told reporters in New York after meetings with Saudi Arabia's allies.

"There are two options for a settlement in Syria. One option is a political process where there would be a transitional council," he said, describing this as the "preferred option."

"The other option is a military option, which also would end with the removal of Bashar al-Assad from power," Mr Jubeir warned.

"This could be a more lengthy process and a more destructive process, but the choice is entirely that of Bashar al-Assad."

Mr Jubeir would not be drawn on specifics of what a military option would look like, but noted Saudi Arabia was already supporting "moderate rebels" in their battle against Mr Assad.

"Whatever we may or may not do we're not talking about," he said.

"There is a Free Syrian Army that is fighting against Bashar al-Assad.

"There is a moderate Syrian opposition that is fighting against Bashar al-Assad and this opposition is getting support from a number of countries, and we expect that this support will continue and intensify."

US president Barack Obama and Russian president Vladimir Putin have clashed over whether Bashar al-Assad should retain power in Syria. ( UN Photo: Amanda Voisard )

Mr Jubeir said the best option would be for Mr Assad to accept the principles of the Geneva I agreement signed at a peace conference in 2012, laying the groundwork for a transitional government.

Under this plan, he said, Mr Assad would immediately cede power to an executive council made up of both members of his regime and opposition figures.

Russia's president Vladimir Putin and his US counterpart Barack Obama met on Monday and agreed to look for a diplomatic end to the Syrian civil war but clashed over whether Mr Assad should retain power.

The two leaders agreed their armed forces should hold talks to avoid coming into conflict in Syria.

Mr Putin said Russia was pondering what more it could do to support Syrian government and Kurdish forces against Islamic State militants but ruled out deploying Russian ground troops.

AFP