There are also more immediate hurdles. Syrian rebel groups were due to hold talks with Turkish officials in Ankara on Thursday.
A senior rebel official told Reuters this week the groups were discussing with Turkey the ceasefire proposal being negotiated with Russia.
They had rejected Moscow's demand to exclude a rebel stronghold near the capital from any deal, said Munir al Sayal, the head of the political wing of Ahrar al Sham, whose group is involved in talks with Turkey.
Ankara supports the Free Syrian Army, a loose alliance of rebel groups, some of which it is backing in operations in northern Syria designed to sweep Islamic State and Syrian Kurdish fighters from its southern border.
The United States is backing the Syrian Kurdish YPG in the fight against Islamic State in Syria, a move that has infuriated Turkey, which sees the YPG as an extension of the militant Kurdistan Workers Party (PKK). Ankara fears that advances by Kurdish fighters in Syria could inflame militants at home.
Turkish President Tayyip Erdogan has accused the United States of supporting terrorism in Syria, including Islamic State, comments that Washington has dismissed as "ludicrous".
"We, as Turkey, have been calling to Western nations for some time to not distinguish between terrorist organizations and to be principled and consistent in their stance," Erdogan said in a speech on Thursday.
"Some countries, namely the United States, have come up with some excuses on their own and overtly supported the organisations that massacre innocent people in our region. When we voice these, these gentlemen are bothered by it."