The forces of Bashar al-Assad are thought to be preparing an assault on Syria's biggest city after capturing the strategically important town of Qusair with large-scale backing from Lebanon's Hizbollah.
The rebels' loss of the town - which holds symbolic value for both sides in the country's vicious civil war - after three weeks of bitter fighting comes amid reports that the regime is readying an attempt to retake Aleppo.
Such an attack could come with further help from Hizbollah; some commanders of the Shia militia claim they are ready to take thousands of men across the border.
The opposition holds about half of the northern city, and a battle to retake it is likely to be bloody.
Large swathes of territory remain outside the control of the regime 2 years after the start of the uprising. But recent gains on the ground have put Assad in a stronger position.
UN international envoy for Syria Lakhdar Brahimi said the delayed peace talks might happen in July. There is deep apprehension that the talks failing, or not taking place, would accelerate the spread of strife from Syria into neighbouring countries.
Residents in the predominantly Shia suburb of Dahiyah in Beirut celebrated after news broke of the fall of Qusair. The movement's leader, Hassan Nasrallah, has given unequivocal backing to the Assad regime.
Furthermore, the entry of the Shia militia will further demarcate the war along sectarian lines between the Alawites, a Shia offshoot from which Assad and the ruling elite are drawn, and the mainly Sunni opposition.
Anxiety to keep the Geneva talks on track is seen as one of the reasons why the Americans are urging caution after the latest claim, by France and Britain, that the regime has been using sarin gas. Officials in Washington stated that stronger evidence would be needed to prove that Assad had crossed the "red line" on chemical weapons set down by President Barack Obama.
Humanitarian organisations called for help for victims of conventional weapons in Qusair. Doctors said there was little or no medicine left for the severely injured who could not leave when much of the population fled the fighting.
The rebels have now lost their supply line to the Lebanese border 10km away, and Sunni villages in the area are more vulnerable to the actions of the regime and Hizbollah.