Syria's opposition called Friday on President Bashar al-Assad to prove it is working for a transition of power in the war-torn country, as they gathered in Istanbul to discuss a US-Russian initiative for peace.

"We want to stop the bloodshed. It's very important for us to have goodwill gestures, and from both sides," Khaled al-Saleh, spokesman for the Western-backed National Coalition -- the main opposition group -- told reporters in Istanbul.

"We want to make sure that when we enter those negotiations the bloodshed in Syria will stop," he added.

The call comes hours after key Assad backer Moscow said the Syrian regime is "in principle" willing to join the peace conference proposed by the United States and Russia dubbed "Geneva 2".

Saleh also renewed accusations against the regime over the alleged use of chemical weapons -- this time in the town of Adra near the capital -- "the night before" Moscow said Damascus is ready to join proposed peace talks tentatively set for June.

"In terms of us making goodwill gestures, we're ready to make those," said Saleh.

"From the regime's perspective, at least don't use chemical weapons the night before" Moscow announced Damascus' willingness to go to the negotiating table, he added.

"You know, simple things like that. Stop using Scud missiles. Withdraw the army from Syrian cities."

The opposition group was meeting in Istanbul for a second day to debate whether to take part in Geneva 2.

Other difficult issues on the Coalition's agenda include choosing a new president to replace Ahmad Moaz al-Khatib, who resigned in March, establishing a rebel interim government and expanding the opposition to include new members.

The United States and Russia, which support opposite sides in Syria's raging conflict, proposed the peace conference earlier this month.

The opposition has since held meetings with representatives from key backers and allies Qatar, Saudi Arabia, Turkey, the United States and France, said Saleh.

"We're listening to our partner countries... What I can say, and this is my own impression, is that the international community wants us to form a vision we're all united around, and we're starting to work through that," he added.

But the opposition is itself divided over whether to enter into talks with the regime.

Some within the Coalition said it should negotiate if talks lead to Assad's departure, while others have expressed reservations.