Fighting around the capital city has intensified over the past week, and Western officials have begun speaking about faster change on the ground in a 20-month-old conflict that has killed 40,000 people.
But Russia and the United States, the superpowers that have backed the opposing sides in the conflict, both played down the chance of a diplomatic breakthrough after talks aimed at resolving their differences.
"I don't think anyone believes that there was some great breakthrough," U.S. Secretary of State Hillary Clinton said of a meeting with Russia's Foreign Minister Sergei Lavrov and international mediator Lakhdar Brahimi.
"No one should have any illusions about how hard this remains. But all of us, with any influence, need to be engaged with Brahimi for a concerted, sincere push."
Lavrov said the sides had agreed to send officials to another meeting with Brahimi, but also sounded a sceptical note.
"I would not make optimistic predictions ... It remains to be seen what will come out of this," he added, noting that Brahimi knows the chance of success is "far from 100 percent".
Rebels, meeting in Turkey in the presence of Western security officials, elected a 30-member unified military command, giving prominent posts to Islamists and excluding some senior officers who defected from Assad's army.
Washington and its NATO allies want to see Assad removed from power. Moscow has blocked action against him at the U.N. Security Council, and while outsiders repeatedly point to signs of Russia losing patience with him, its stance has not changed.
The past week has brought a war previously fought mainly in the provinces and other cities to the threshold of the capital.
Cutting access to the airport 20km from the city centre would be a symbolic blow. The rebels acknowledge the airport itself is still in army hands, but say they are blockading it from most sides.
"The rebel brigades who have been putting the airport under siege decided yesterday that the airport is a military zone," said Nabil al-Amir, a spokesman for the rebels' Damascus Military Council.
"Civilians who approach it now do so at their own risk," he said. Fighters had "waited two weeks for the airport to be emptied of most civilians and airlines" before declaring it a target, he added.
He did not say what they would do if aircraft tried to land. Foreign airlines have suspended all flights to Damascus since fighting has approached the airport in the past week, although some Syrian Air flights have used the airport in recent days.
Syria says the army is driving rebels back from positions in the suburbs and outskirts of Damascus where they have tried to concentrate their offensive. Accounts from rebels and the government are impossible to verify on the ground.
"SOME FIGHT LEFT IN THEM"
Although Western opponents of Assad believe events are tipping against him, they also acknowledge that the war is still far from over.
"It's very clear to me that the regime's forces are being ground down," U.S. ambassador to Syria Robert Ford, withdrawn last year, was quoted as saying by CNN. "That said, the regime's protection units continue to maintain some cohesion, and they still have some fight left in them, even though they are losing. I expect there will be substantial fighting in the days ahead."
Rami Abdelrahman, of the Syrian Observatory for Human Rights, which has tracked the fighting since it began in March 2011, said: "I think it's unrealistic to expect that the battle is in its last stages right now."
The meeting of rebels in Antalya, Turkey, was aimed at forming a structure to run the conflict in conjunction with a new opposition National Coalition, which some European and Arab states have recognised as Syria's legitimate representatives.
One delegate at the meeting, who asked not to be identified, said two-thirds of the 30 members of the newly named command had ties with the Muslim Brotherhood or were its political allies.
"We are witnessing the result of the Qatari and Turkish creations," said the delegate, referring to leading anti-Assad countries that are seen as backing the Brotherhood.
Colonel Riad Asaad, founder of the Syrian Free Army rebel force, and General Hussein Haj Ali, the highest-ranking officer to defect from Assad's military, were among those excluded.
NATO decided this week to send U.S., German and Dutch batteries of air-defence missiles to the Turkish border, putting hundreds of American and European NATO troops close to the frontier with Syria for the first time in the crisis.
Russia's ambassador to NATO said the move risked dragging the alliance into the conflict.
"This is not a threat to us, but this is an indication that NATO is moving toward engagement, and that's it," Alexander Grushko said. "We see a threat of further involvement of NATO in the Syrian situation as a result of some provocation or some incidents on the border, if they take place."
The Dutch on Friday said they would send two Patriot batteries with up to 360 personnel. Germany approved its mission on Thursday.
The United States and its NATO allies have issued coordinated warnings in recent days to Assad not to use chemical weapons, prompting Syria to accuse Western countries of conjuring the threat to justify a military intervention.
Syria has not signed an international chemical weapons treaty banning poison gas, but has repeatedly said that it would never use such weapons on its own people.
United Nations Secretary General Ban Ki-moon said: "We have no confirmed reports on this matter. However, if it is the case, then it will be an outrageous crime in the name of humanity."