Washington stuck to its demand on Thursday that Syrian President Bashar al-Assad leave power, ahead of peace talks which will include Assad's main ally Iran for the first time, reflecting his stronger position since Russia joined the war on his side.

Throughout four years of war that has killed 250,000 people and driven more than 10 million from their homes, Assad's main ally Tehran was locked out of a succession of international peace conferences, all of which ended in failure.

But four weeks after Russia began bombing Assad's enemies on the ground, the countries that demand he leave office, including the United States, European powers and Saudi Arabia, have agreed to give Iran a seat at the negotiating table.

"Those who tried to resolve the Syrian crisis have come to the conclusion that without Iran being present, there is no way to reach a reasonable solution to the crisis," Iran's Foreign Minister Mohammad Javad Zarif said on his arrival in Vienna on Thursday ahead of Friday's conference.

Zarif met U.S. Secretary of State John Kerry on Thursday in Vienna for talks on other issues including the July nuclear agreement between Iran and global powers. Kerry also met Russia's Sergei Lavrov and the foreign ministers of Saudi Arabia and Turkey.

State Department counsellor Tom Shannon said in Washington Kerry would use the conference to see whether Tehran and Moscow were willing to accept a change of leadership in Damascus, and also gauge their commitment to fighting the Islamic State group.

Kerry would assess the extent to which Iran and Russia "are prepared to work broadly with the international community to convince Mr Assad that during a political transition process he will have to go," Shannon said.

The United States and its European and Middle Eastern allies have demanded Assad agree to leave power as part of any peace deal. He refuses to go, and Russia and Iran have consistently rejected any such demands.

Russia's 4-week-old air campaign on Assad's behalf, which has been accompanied by an Iranian-backed ground offensive, makes the prospect that Assad's insurgent foes can force him out of power on the battlefield look more remote than ever. Some Western officials have spoken lately of temporary arrangements under which Assad could remain for a certain period.

Western officials have played down hopes for progress at this week's talks, while nevertheless suggesting that the meeting provides an opportunity to test whether there is flexibility in the Iranian and Russian positions.

TRANSFORMED

Russia's surprise decision to join the war a month ago has transformed the situation on the battlefield as well as at the conference table.

The United States is leading its own air campaign against militants from Islamic State, the world's most violent jihadist group, which controls swathes of eastern Syria and northern Iraq. Russia says Islamic State is its target too.

But despite having the same professed enemy, Washington and Moscow have very different friends and opposing views of Syria.

Washington says Assad's presence makes the situation worse by encouraging militants to fight him, and it is supporting what it describes as "moderate" rebels.

While Russia says its own bombing campaign targets Islamic State, the overwhelming majority of its air strikes have hit other groups opposed to Assad, including many that are supported by Washington's allies.

Kerry said on Wednesday that Washington was stepping up its diplomacy to end the Syrian conflict, even as it increases support for moderate rebels.

German Foreign Minister Frank-Walter Steinmeier said in Athens on Thursday that it would be a success if the countries participating in Friday's talks could agree on some basic principles, such as maintaining Syria's territorial integrity and a process for creating a transitional government.

"The breakthrough will not come tomorrow," he said.

Iran has shown no signs it is ready to dump Assad. A senior Iranian official told Reuters there was no candidate to replace Assad, describing him as the only one who can prevent Syria from collapse. He added that the priority was to help Assad defeat Islamic State.

"We have been helping Syria on this matter and will continue to do so as long as it is needed by the government," he said.

Neither Syria's main political opposition body, which has objected to Iran's participation, nor representatives of the armed opposition were invited to the meeting.

Assad's government has yet to comment on the talks.