Syria's main opposition National Coalition on Wednesday said any political settlement to the country's two-year-old conflict must start with President Bashar al-Assad's ouster, implicitly rejecting a US-Russian initiative for dialogue with the regime.
UN-Arab League peace envoy Lakhdar Brahimi, meanwhile, hailed the agreement between Washington and Moscow to push both sides in the Syrian conflict to end the bloodshed and sit down for talks.
"The National Coalition welcomes all international efforts which call for a political solution to achieve the aspirations of the Syrian people and their hope for a democratic state, so long as they begin with the departure of Bashar al-Assad and his regime," the opposition umbrella group said.
The response could be a blow to the US-Russian initiative, which is based on an international deal agreed in Geneva last year that makes no mention of Assad stepping down.
The opposition has long insisted that the embattled president cannot stay on, but the regime insists that Assad's future will be decided in elections, with a presidential vote scheduled for 2014.
"This is the first hopeful news concerning that unhappy country in a very long time," Brahimi said of the deal announced by US Secretary of State John Kerry and his Russia counterpart Sergei Lavrov after talks in Moscow on Tuesday.
"The statements made in Moscow constitute a very significant first step forward. It is nevertheless only a first step," said Brahimi, who an aide said has been mulling resignation over the apparent absence of a political track to resolve a war that has killed more than 70,000 people.
"There is every reason to expect" backing for the accord from the remaining UN Security Council permanent members" -- Britain, China and France, the veteran Algerian diplomat added.
"It is equally important that the entire region mobilises in the support of the process."
British Prime Minister David Cameron announced on Wednesday he will fly to the Russian resort of Sochi on Friday to discuss the Syrian conflict with President Vladimir Putin.
The US-Russian deal was announced at a joint press conference, with Lavrov saying the two countries were ready to use all their resources to bring "the government and opposition to the negotiating table."
"We agreed that Russia and the United States will encourage both the Syria government and opposition groups to find a political solution," he said.
Lavrov and Kerry said they hoped they could convene an international conference by the end of May to build on a deal agreed by world powers in Geneva last June for a peaceful solution in Syria.
The Geneva deal, which calls for a transitional government but makes no mention of Assad's fate, "should be the roadmap... by which the people of Syria can find their way to the new Syria and in which the bloodshed, the killing, the massacres can end", Kerry said.
"The alternative is that there's even more violence; the alternative is that Syria heads even closer to the abyss, if not over the abyss and into chaos," he warned.
Kerry said only the Assad regime and the opposition can determine the make-up of a transitional government to shepherd the war-torn nation towards democratic elections.
"It's impossible for me as an individual to understand how Syria could possibly be governed in the future by the man who has committed the things that we know have taken place," Kerry said.
"But I'm not going to decide that tonight, and I'm not going to decide that in the end."
Russia has long accused the West of aggravating the Syrian conflict by seeking to topple Assad, whose regime has yet to react to the initiative announced in Moscow.
And the US and other Western states have accused Russia of failing to use its influence with the regime to halt the bloodshed, and of keeping up military deliveries to Assad.
UN efforts were under way on Wednesday to free four peacekeepers from the Philippines who were seized by a rebel group on the Golan in the second such abduction of Filipino forces in two months.
Manila called their detention a "gross violation" of international law and urged the Security Council to "use its influence for the early and safe release" of the four.
On the ground, the Syrian Observatory for Human Rights said rebels shot down a regime fighter jet over the northern province of Aleppo.
The Observatory also reported that regime troops had stormed the rebel-held town of Khirbet Ghazaleh in southern Daraa province, taking large parts of it, adding that fighting was ongoing.
In other developments, the Internet was restored in Syria, residents and state media said, after a two-day blackout that official media blamed on a technical fault.