Meeting in Riyadh aims to unite opposition groups before potential talks with the Syrian government.
Hundreds of Syrians, including rebel fighters, have left the last rebel-held area of Homs as part of a rare local ceasefire deal negotiated between the opposition and the government.
The fighters and their families were being moved to rebel-held areas of the northwest near the Turkish border on Wednesday.
Homs was long a centre of the uprising against President Bashar al-Assad.
The deal comes after a major Syrian army ground offensive to the north of the city backed by Russian air strikes.
Several buses left the al-Waer district early on Wednesday and others were queuing up to leave on its outskirts, witnesses said.
The city’s governor Talal al-Barazi told reporters that 300 fighters were on board, together with 400 members of their families. The fighters took light weapons with them, he said.
The deal echoes a local ceasefire agreed in September elsewhere in Syria under which rebel fighters were supposed to be transferred to Idlib, though it has yet to be fully implemented.
‘No one is winning’
Al Jazeera’s senior political analyst, Marwan Bishara, said there is a “certain will on both sides to calm things down, especially in places where it is clear that one party has the upper hand over the other”.
“The general mood is; no one is winning in Syria, so let’s – at least for the time being – settle down, take it easy, until we see if there is a serious transitional process possible.”
The evacuation of fighters and their families was the first phase of the agreement.
“They will head to opposition-controlled areas further north. The government in turn will lift the siege of the district and end military operations there,” Al Jazeera’s Zeina Khodr said.
“But the warring sides seem to be interpreting other details differently. The government says the deal means al-Waer will return to state control, cleared of weapons, and fighters who chose to stay will have their legal status settled. It also said that all rebels would leave the district within two months,” Khodr added.
“The opposition, however, denies the district will return to government control.”
He described the rebels who left as “militants who reject the agreement”, saying they would leave with their families.
“The Waer neighbourhood arrangement will be completely safe and there will be no weapons in Waer after the implementation of the agreement,” he said, adding security forces would go back to work in the area and would be the only ones armed.
The UK-based Syrian Observatory for Human Rights monitoring group said about 750 people were expected to leave during the day for rebel-held areas in Hama and Idlib provinces.
The United Nations is presiding over implementation of the deal, which was agreed directly between the Syrian sides.
Some diplomats say local ceasefires may be the most effective way of gradually bringing peace to a country where more than 250,000 people have been killed, though one concluded in Homs in 2014 was widely seen as a forced surrender.
Syria peace talks involving world powers in Vienna in October called for a nationwide ceasefire and a renewal of UN-brokered talks between the rival Syrian sides.
Meanwhile, Russia said Moscow, Washington and the United Nations will hold Syria talks in Geneva on Friday as diplomatic efforts to end the conflict are ratcheted up.
“We will be having consultations in the trilateral format – Russia, the United States and the UN,” Deputy Foreign Minister Gennady Gatilov told RIA Novosti state news agency on Wednesday.
In another diplomatic track, Syrian opposition groups were meeting in Saudi Arabia to agree on a transitional government that does not include Assad.
The closed-door talks are seen as the most serious effort yet to unify the opposition, a step considered vital to peace talks sought by world powers but which has angered Iran.
In a separate meeting in Riyadh, Gulf leaders gathered on Wednesday for an annual summit in the face of plunging oil revenues, the war in Yemen and pressure for peace in Syria.
The kings and emirs were expected to voice support for the unification of Syria’s opposition.