Syria's main opposition National Coalition said on Wednesday any political settlement to the country's two-year-old civil war must start with President Bashar al-Assad's ouster, implicitly rejecting a US-Russian initiative for dialogue with the regime.
But UN-Arab League peace envoy Lakhdar Brahimi has hailed the agreement between Washington and Moscow to push both sides in the Syrian conflict to end the bloodshed and sit down for talks.
And on the ground, the head of the powerful jihadist rebel Al-Nusra Front, Abu Mohammed al-Jawlani, was wounded by army bombing near Damascus, a watchdog said.
"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 vote scheduled for 2014.
Speaking of the deal announced by US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov after talks in Moscow on Tuesday, Brahimi said "this is the first hopeful news concerning that unhappy country in a very long time."
"The statements made in Moscow constitute a very significant first step forward. It is nevertheless only a first step" toward ending a war that has killed more than 70,000 people, Brahimi said.
"There is every reason to expect" backing for the accord from the remaining UN Security Council permanent members" -- Britain, China and France, he added.
"It is equally important that the entire region mobilises in the support of the process."
British Prime Minister David Cameron announced he will fly to Russia on Friday to discuss the Syrian conflict with President Vladimir Putin.
In announcing the deal, Lavrov said 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.
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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, "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.
Kerry said only the 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.
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.
On the ground, the Syrian Observatory for Human Rights said Al-Nusra chief Jawlani and some of his group had been wounded south of Damascus.
Observatory chief Rami Abdel Rahman was only immediately able to say that Jawlani had been hit in the foot.
Al-Nusra, which seeks to establish an Islamic state in Syria, is among the most prominent and successful fighting organisations in the conflict.
Elsewhere, rebels shot down a air force fighter jet over the northern province of Aleppo and the Observatory also reported that troops had stormed the rebel-held town of Khirbet Ghazaleh in southern Daraa province, taking large parts of it.
Meanwhile, UN efforts were underway to free four peacekeepers from the Philippines seized on Tuesday by a rebel group on the Golan in the second such abduction of Filipino forces in two months.
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.
And the State Department said the United States is to donate another $100 million (76 million euros) in humanitarian aid to Syrian refugees, boosting its total to $510 million.
Since the war erupted, more than 1.5 million Syrians have fled into neighbouring nations, including Turkey, Jordan and Lebanon, vastly straining their resources.