Syrian opposition has sought support for a transitional government at an international Friends of Syria conference in Istanbul, as the US prepares new non-lethal military aid for Syrian rebels.

US Secretary of State John Kerry, Germany's Westerwelle and British Foreign Minister William Hague were greeted on Saturday by Turkey's Foreign Minister Ahmet Davutoglu at the start of the meeting.

The meeting of 11 countries from the Friends of Syria alliance comes after the al-Nusra Front, among the strongest formations seeking to topple Syrian President Bashar al-Assad, pledged allegiance to al-Qaeda leader Ayman al-Zawahri on April 10.

A Syrian rebel leader said only force could end the two-year conflict and ruled out the possibility of any negotiation with Assad's administration other than over its exit.

"There is going to be no solution through negotiations with this regime. This will not be settled other than by force, maybe in the final stages when the regime feels it has lost everything, it might want to negotiate for its exit," Brigadier Selim Idris told reporters on the sidelines the conference.

Aid package

Kerry is expected to announce a new aid package, which would mark a recalibration of US policy toward rebel groups in the Syrian civil war.

The US plans to provide about $130m in new non-lethal aid to the Syrian opposition that could include for the first time battlefield support equipment such as body armour and night-goggles.

Davutoglu said that he hoped the meeting will help bring peace to Syria.

"We are all working together to end the pains of the Syrian people which lasted more than two years now and more than 100,000 people being killed," he said.

Among those invited to Istanbul will be Moaz Alkhatib, a moderate cleric from Damascus, who said he was resigning as head of the Syrian National Coalition in March after other members of his proposal for negotiating with Assad.

Envoys representing most of the 11 Friends of Syria countries met in Cairo this month to press Alkhatib to stay on as leader of the coalition, a 60-opposition umbrella group backed by the West and Gulf Arab states.

Dozens of leftist protesters had earlier staged a demonstration outside the venue where the meeting was being held.

They set effigies of Kerry and Davutoglu on fire.