Chief diplomats from the U.S., Russia, Saudi Arabia and Turkey will come together in Vienna to discuss Syria on Friday, as the ongoing crisis deepens following airstrikes conducted by Moscow and debates on Syrian President Bashar Assad's future have become more palpable regarding the transition government plan on the table.U.S. Secretary of State John Kerry, Russian Foreign Minister Sergey Lavrov, Saudi Foreign Minister Adel al-Jubeir and Turkish Foreign Minister Feridun Sinirlioğlu will be the participants at the critical quartet meeting. The meeting comes after Assad's surprise visit to Moscow on Tuesday in his first foreign visit since the outbreak of civil war in 2011, while airstrikes by Russian jets on Syrian opposition targets continue. Accordingly, Assad's future is highly likely to its place on the diplomats' agenda. Turkey and Saudi Arabia are in favor of Assad leaving the office after a temporary process in which he rules the country over a six-month period at most. Previously on Wednesday, Prime Minister Ahmet Davutoğlu eyed a possible six-month transition period with Assad and said, "We need to dwell on formulas in which Assad leaves. I wish Assad could have stayed longer there [in Moscow], and the Syrian people could feel at ease, then the real transition period would have started," referring to Assad's surprise visit to Moscow.A senior diplomat also told a group of journalists on Monday that Ankara is ready to accept a transition period in which Assad remains in symbolic power for six months before leaving office. "Work on a plan for Assad's departure is underway. ... [Assad] can stay for six months and we accept that because there will be a guarantee of his departure," he said. Ankara's arguments have also been mirrored by Riyadh, as al-Jubeir suggested in the joint meeting with his German counterpart Frank Walter Steinmeyer. Indicating that Assad can hold onto power in war-torn Syria only until a transitional council of opposition and government figures has been set up, he said, "We have said that... after the formation of the governing council Bashar Assad will have to leave.""Whether it's today, whether it's a week, whether it's a month, it's up to the Syrian people, but from the very beginning it's clear that Bashar Assad has no future in Syria", Al Jubeir said.Besides, Washington thinks that ousting Syrian leader Bashar Assad is essential for fighting the Islamic State of Iraq and al-Sham (ISIS) with the moderate Syrian opposition. In a previous televised interview, U.S. President Barack Obama said, "And what we've learned is that as long as Assad remains in power, it is very difficult to get those folks to focus their attention on [ISIS]," adding that he was "skeptical from the get-go" about the notion of creating an army of moderate forces in Syria.Yet, Washington's inconsistent policies in Syria since the beginning of the crisis has allowed Moscow to maneuver in favor of staunchly supporting Assad. Although two capitals apparently agreed on the anti-ISIS fight, Russian jets' killing of moderate opposition groups, fighting regime forces and even civilians, has stirred up tensions. Russian airstrikes in Syria have reportedly killed 370 people since they began on Sept. 30, around a third of them civilians, a monitoring group said Tuesday.