Saudi Arabia has said Iran must accept the removal of President Bashar al-Assad as part of any solution to the conflict in Syria.
Saudi Foreign Minister Adel al-Jubeir made the remarks as talks among international foreign ministers on the crisis get under way in Vienna.
Iran is for the first time taking part in such talks, which will also include Russia and Turkey.
Russia and Iran both support Syrian President Bashar al-Assad.
They have both recently stepped up their military role in the conflict.
The US, Turkey, Saudi Arabia and other Gulf Arab nations have long insisted Mr Assad cannot play any long-term role in Syria’s future.
Mr Jubeir told the BBC that there was “no doubt” Mr Assad had to go. “He will go either through a political process or he will be removed by force,” he said.
Earlier American Secretary of State John Kerry said the US was intensifying diplomatic efforts to end the “hell” of Syria’s civil war even as it increases support for moderate rebels.
Foreign ministers from the UK, France, Germany, Egypt, Lebanon and the EU have also confirmed they will attend the meeting, and other Middle Eastern powers are also expected.
In a speech at the Carnegie Endowment for International Peace think-tank in Washington, ahead of the talks, Mr Kerry said:
“At the end of the day, nothing would do more to bolster the fight against Daesh [the Islamic State militant group] than a political transition that sidelines Assad so that we can unite more of the country against extremism.”
But in his speech, Mr Kerry stressed that the US and Russia also shared “common ground”, arguing that both want “a united, secular Syria”.
Iran is believed to have spent billions of dollars over the past four years propping up Mr Assad’s government, providing military advisers and subsidising weapons.
However, Syria’s political opposition has warned that Iran’s involvement will only complicate the meeting in Vienna.
Iran has long acknowledged sending military advisers to Syria, but has denied the presence of any ground forces. Despite that, unconfirmed reports earlier this month said that hundreds of Iranian troops had arrived in Syria.
They were reported to be joining government forces and fighters from the Lebanese Shia Islamist movement, Hezbollah, in assaults on rebel positions in northern and central Syria.
Russia began its military intervention in Syria at the end of last month, launching air strikes in support of Mr Assad.
Washington has accused Moscow of concentrating its air campaign in Syria on moderate opposition groups rather than on the Islamic State militant group.
