Syria's main opposition emphasized on Thursday its demand that head of Syrian regime Bashar al-Assad assumes no part in an interim period under any UN-sponsored peace deal, despite hypothesis it could ease its stance on account of Assad's military strength.

More than 140 members from a wide range of Syria's mainstream opposition held a gathering in Saudi Arabia, seeking to unify their stance ahead of UN-backed peace talks after two years of Russian military intervention that backed Assad's recapture of Syria's main cities.

"The Syrian opposition has sent a message that it is ready to enter serious direct talks over a political transition in Syria and has a unified position and a vision for the future of Syria," Reuters quoted opposition spokesman Ahmad Ramadan as saying.

An opposition communique said the participants supported a UN-based process that would allow Syria to undergo "a radical political transition" from an "authoritarian system" to a democracy where free elections would be upheld.

They also blasted Iran's military presence in Syria and called on Iran-backed militias to leave the country, the communique added, underlining that these militias sowed "terrorism and sectarian strife" between Sunni and Shi'ite Muslims.

UN peace talks mediator Staffan de Mistura, preparing for a new round of Geneva talks, will visit Moscow on Friday, where he is expected to discuss the situation in Syria. Russian President Vladimir Putin has called for a congress of the Syrian government and opposition to draw up a framework for the future structure of the Syrian state, adopt a new constitution and hold elections under UN supervision.

But he has also said that any political settlement in Syria would be finalized within the Geneva peace talks process overseen by the United Nations. The communique also said that negotiations should be direct and without preconditions based on past UN Security Council resolutions.

The opposition backed the restructuring of the army and security organs and preservation of state institutions, but called for the trial of those responsible for war crimes. The meeting, which included independents and Free Syrian Army military factions, also blamed the Syrian government for the lack of progress in Geneva-based talks held in the past.

"The political process has not achieved its goal because of the regime's violations," the communique said, citing the bombing of civilian areas, the siege of rebel-held areas and the detention of tens of thousands of dissidents. Syria's civil war, now in its seventh year, has killed hundreds of thousands of people and created the world's worst refugee crisis, driving more than 11 million people from their homes.