US Secretary of State John Kerry called Thursday for Syrian and Arab ground troops to take on Islamic State militants to enable a complete defeat of the organisation.
"I think we know it, that without the ability to find some ground forces that are prepared to take on Daesh (ISIS), this will not be won completely from the air," Kerry told delegates a gathering of dozens of foreign ministers in Belgrade.
When asked later whether he was referring to Western or Syrian ground forces, he said: "Syrian and Arab, as we have been consistently". Washington is however due to send 50 US special forces into Syria soon.
His comments at the annual ministerial council of the Organisation for Security and Cooperation in Europe (OSCE) came after Britain's parliament voted late on Wednesday to join the US-led bombing campaign over Syria.
The last round of Syria peace talks were held in Vienna last month, bringing together 17 countries including Russia, the United States, Saudi Arabia and Iran.
The talks set a fixed calendar for a ceasefire followed by a transitional government in six months and elections one year later, but Syrian opposition figures have called this unrealistic.
"If we get the political transition in place we empower every nation and every entity to come together, the Syrian army together with the opposition... together with Russia, the United States and others to go and fight (against) Daesh," Kerry said in Belgrade.
"Just imagine how quickly this scourge could be eliminated in a matter of literally months if we were able to secure that kind of political resolution."
In Brussels on Wednesday, Kerry had urged NATO allies to intensify the fight against the ISIS group.