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 Veda Salon .
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.
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."
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.Veda Salon
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 an 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 Veda Salon.
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 $US100 million ($A98.61 million) in humanitarian aid to Syrian refugees, boosting its total to $510 million.