A source close to Hezbollah said on Sunday that pro-Assad forces held "80 percent" of Qusayr, which has been in rebel hands for more than a year.
"I think the fact that the situation on the ground is so bad should make these Coalition members start sweating," said a high-level source close to the rebel Free Syrian Army.
"The rebels are resisting but the onslaught is massive," added the source.
Also increasing the pressure on the opposition to put aside their differences is a top-level Paris meeting on Monday between the foreign ministers of Russia, France and the United States.
On the same day, EU foreign ministers will meet in Brussels to discuss whether to lift an arms embargo, allowing weapons to flow to the rebels fighting Assad.
An EU embargo on Syria expires at the end of May but Britain and France are pressuring other EU member states to lift it.
The opposition Coalition "had better have a decision" on whether to attend the so-called Geneva 2 peace talks before the Brussels meeting starts, said the source.
While the opposition dithering extended into an unscheduled fourth day of talks, Syria Foreign Minister Walid Muallem announced on Sunday a decision "in principle" to participate in the talks.
The main point of division in the Syrian opposition, which relies on foreign backing for survival, revolves around whether to include new members in the group.
Some have raised objections to the inclusion of veteran Christian dissident Michael Kilo, accusing him of being backed by Saudi Arabia and of trying to play off Qatar's influence over the group.
Though the secular Kilo would bring in several new women and members of Syria's religious minorities, opponents say his entry would shrink the influence of the Muslim Brotherhood and force Saudi control on the coalition.
"You have Saudi Arabia and the United Arab Emirates pushing to include up to 30 new members in the National Coalition. Their goal is to downsize the Muslim Brotherhood's influence over the group," a Coalition member told AFP on condition of anonymity.
"The issue is about much more than just names. It's about which states want to control the opposition," the rebel army source said.
A Western diplomat, who also asked not to be named, said it was important the Coalition expand, not just because that would bring in more women but also because it would avert "the alienation of certain donor countries" such as Saudi Arabia.
Getting agreement is "crucial" before the Paris and Brussels meetings kick off on Monday, the diplomat said, "so that we can show the Syrian opposition is united."
Another diplomatic source said a solution needed to be reached "to be able to bring together a delegation that is up to the task" if negotiations begin between rebels and the regime.
But outside the Istanbul hotel playing host to the talks, a group of some 15 protesters gathered, enraged by the Coalition's failure to clinch agreement.
"We want them to decide. They've been in there four days, discussing expansion while they should be taking a stand against Iran and Hezbollah's attack on Qusayr. What are they doing?" said one activist from Daraa in Syria's south.
Another rejected the idea of talks outright.
"We don't even want negotiations. We just want weapons. We don't want to negotiate with the criminal regime," said one young activist from Damascus who identified himself as Motasem.
"If the international community does not want to intervene directly, we'll get rid of him ourselves. We just want weapons."