United Nations (United States) (AFP) - Syrian leader Bashar al-Assad must leave office or face being turfed out by force, Saudi Foreign Minister Adel al-Jubeir said, rejecting Russia's bid to build support for its ally.

Speaking in New York after meeting Saudi Arabia's allies, Jubeir on Tuesday dismissed Russia's call for a coalition to defend Assad against the Islamic State group as a "non-starter."

He warned that other countries would step up support for rebels from Syria's moderate opposition, leaving Assad with no choice but to step down or face what he called the "military option."

And he scorned Iran's involvement in Russia's putative alliance, describing Tehran as an "occupying power" in Syria and accusing it of fomenting terrorism and extremism across the region.

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

He spoke of only two possible outcomes for a settlement in Syria, saying a transitional council reached through a political process would be the "preferred option."

A second, military option "could be a more lengthy process and a more destructive process, but the choice is entirely that of Bashar al-Assad," the Saudi foreign minister said.

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

"Whatever we may or may not do we're not talking about," he said, but quickly added: "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," he noted.

"And we expect that this support will continue and intensify."

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

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- 'Sail into the sunset' -

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

"And, sometime between the formation of this council and elections -– whether it's a day or a week or a month, I don't know -– President Assad would sail into the sunset," he said.

Russia's President Vladimir Putin and Iran's President Hassan Rouhani came to the UN General Assembly in New York this week to urge the world to support Assad and defeat the Islamic State.

Putin proposed a Security Council resolution to govern foreign military action in Syria, but Saudi Arabia, a key member of the existing US-led coalition against the IS group rejects this.

"I think if the Russians were serious about fighting Daesh, they could join the existing international coalition," Jubeir said, using the Arabic acronym for the Islamic State group.

"But for them to go out and insert forces into Syria... is a big step, and is an indication that their objective may be to prop up the Assad regime more than it is to fight Daesh."

He denied European powers are "going soft" and looking to Russia to resolve the problem and said the United States is only talking to Moscow to avoid chance encounters between their forces.

As to Iran, Jubeir said, the only way it can help is to withdraw.

"Iran is part of the problem and cannot be part of the solution," he said.

"It should withdraw its forces from Syria and withdraw the Shiite militias that it inserted into Syria and then it can talk about a diplomatic solution," Jubeir declared.

"But what we're looking at now is Iran is a occupying force in Syria."

And, asked whether Russia and Iran might support a transition away from Assad's rule, he admitted: "Hope is not that great."