Members of Syria’s opposition coalition are in Istanbul to decide whether they will take part in talks, which aim to bring an end to the country’s civil war.
Washington and Moscow want to hold a conference in Geneva by the end of the month.
The Syrian conflict is said to have claimed at least 80,000 lives.
“We have a lot of unknowns about the Geneva conference,” said Louay Safi, a member of the Syrian National Coalition.
“We are for any conference that helps transition the situation into an elective government away from the dictatorship, but that will be our condition. We are not going to accept any negotiations that do not indicate that Assad is going to be out,” he continued.
Meanwhile some of the fiercest fighting in months has been seen in the town of Qusair, on the Syria-Lebanon border.
The violence has drawn in fighters from Lebanon’s Hezbollah movement. That are signs, according to Britain, that Iran and its allies in the militant group are giving increasing support to Syrian President Bashar al Assad.