Hezbollah fighters spearheaded an assault that cut off Syrian rebel fighters in the central town of Qusayr on Wednesday drawing condemnation from the UN human rights body and a US demand for an immediate withdrawal.
Russia warned a European Union decision to lift its arms embargo on rebels fighting to oust its ally, Syrian President Bashar al-Assad, harmed its joint efforts with the United States to end the conflict.
Hopes are building for a US-Russian initiative for a peace conference to be held in Geneva next month, but serious obstacles could still scupper the talks -- not least divisions within Syria's opposition.
After seven days of talks in Istanbul -- four more than scheduled -- the opposition National Coalition said it would only attend the proposed conference if a string of conditions were met, including Assad's resignation.
In Geneva, 36 of the 47 members of the UN Human Rights Council voted in favour of a resolution that implicitly refers to the involvement of Hezbollah fighters from neighbouring Lebanon in the fierce battle for Qusayr.
The non-binding text put forward by the United States, Turkey and Qatar "condemns the intervention of foreign combatants fighting on behalf of the Syrian regime in Al-Qusayr."
It also expressed "deep concern" that the involvement of the fighters could "further exacerbate the deteriorating human rights and humanitarian situation".
The United States rejected a statement by Russian Foreign Minister Sergei Lavrov that the resolution was "unwholesome" and undermined peace efforts.
"We don't see this as... undermining in any way" but rather an effort to put rights abuses on record and work towards a solution, said US ambassador to the council Eileen Chamberlain Donahoe.
The US State Department demanded the immediate withdrawal of Hezbollah fighters.
"This is an unacceptable and extremely dangerous escalation. We demand that Hezbollah withdraw its fighters from Syria immediately," spokeswoman Jen Psaki said.
The Syrian army said it had seized the disused Dabaa military airfield north of Qusayr, giving pro-Assad forces control of all roads out of the town in a major setback for the besieged rebels.
A military source told AFP the battle for the airfield was fierce and lasted several hours. "The operation led to the liberation of the airport and the deaths of several men who were inside.
"There are bodies littering the ground, rebels have been captured and others surrendered. The army is now advancing on the town of Dabaa," the source added.
Control of Qusayr is essential for the rebels as it is their principal transit point for weapons and fighters from Lebanon while it helps the army consolidate its grip on a key road from Damascus to the coast -- the heartland of Assad's Alawite community.
Hezbollah's Al-Manar television showed live images it said were from Dabaa airfield taken after the army had recaptured the facility.
In Paris, French Foreign Minister Laurent Fabius said there were some 3,000 to 4,000 Hezbollah fighters in Syria, double the 1,700 number previously reported.
The opposition National Coalition is to insist on Assad's resignation and those of his top security chiefs as a precondition for taking part in next month's proposed peace conference, according to a text leaked to reporters.
"The head of the regime must resign, alongside the heads of the military and security forces, who must be excluded from the political process," said the text agreed at the Istanbul talks.
It came after senior envoys from the opposition's main foreign backers put heavy pressure on delegates to agree a common line in the face of mounting disillusion among rebel fighters on the grounds at the wrangling among exiled leaders.
But the idea of Assad stepping down in advance of the proposed conference was swiftly rejected by the regime.
"Do you want the president to resign before the conference, that is not possible," Foreign Minister Walid Muallem told the Beirut-based Arab news channel Mayadeen.
Muallem said the regime still planned to send delegates to the planned peace conference and hoped for a positive outcome.
"If we reach an agreement in Geneva, and I hope we will, it will be put to a referendum and if the people approve what we agreed upon, I can assure you it will be fully respected," he said.
Key Assad ally Iran said it had received "an oral invitation" to attend despite strong opposition from some Western governments, notably France.
The state IRNA news agency quoted Deputy Foreign Minister Hossein Amir Abdollahian as saying "an oral invitation" had been made "but we have not received a written one."
Co-organiser Russia has pushed hard for Iran to be invited to the Geneva conference, insisting it is "a very important outside player."