The regime triumph in Qusair, which Assad's forces had bombarded for months without success, demonstrates the potentially game-changing role of Hezbollah in Syria's civil war. The gain could also embolden Assad to push for all-out military victory rather than participate in peace talks being promoted by the United States and Russia.
The Shiite militant group lost dozens of fighters in the battle for Qusair, underlining its commitment in support of Assad's regime and edging the fight in Syria further into a regional sectarian conflict pitting the Middle East's Iranian-backed Shiite axis against Sunnis.
Most of the armed rebels in Syria are members of the country's Sunni Muslim majority, while Assad has retained core support among the country's minorities, including his own Alawite sect, an offshoot of Shiite Islam, along with Christians and Shiite Muslims.
The overt involvement by Hezbollah, which is heavily invested in the survival of the Damascus regime, has raised tensions considerably in Lebanon, where the militants have come under harsh criticism.
The group openly celebrated Qusair's fall on Wednesday.
The White House on Wednesday night condemned the town's capture and said Hezbollah's involvement threatens Lebanon's stability.
Spokesman Jay Carney said in a statement that Assad's regime couldn't wrest control of the town alone and had to rely on help from Hezbollah. He said Syria's government must allow the U.N. and others to evacuate Qusair's wounded and provide medical treatment.
In the predominantly Shiite northeastern town of Bazzalieh, nsear the Lebanese city of Baalbek, Hezbollah supporters set up a check point, distributing sweets to people and firing in the air in celebration. "Today, we defeated the other Israel," declared Ali al-Bazzal, 23, waving a yellow Hezbollah flag.
In Hezbollah strongholds in southern Beirut, sounds of celebratory gunfire and fireworks rang out for two hours. "Qusair has fallen," read banners hung in the streets.
Both sides had dug in for an all-out battle for Qusair, a key crossroads town of supply lines between Damascus and western and northern Syria that had been under rebel control since early last year.
Over the past two months, the Syrian army has moved steadily against rebels in key battleground areas, making advances near the border with Lebanon and considerably lowering the threat to Damascus, the seat of Assad's government.
Qusair's fall could boost the momentum for Syrian troops in rolling back rebel gains in other parts of central Homs province, as well as in northern Syria, where the sides have been locked in a stalemate for months. Pro-regime media outlets have said government forces are preparing to move to recapture the contested northern city of Aleppo next.
The blow to the rebel movement compounded by deepening divisions in opposition ranks was likely to further discourage it from entering peace negotiations with the regime, which the United States and Russia have been trying to put together in Geneva.
Assad's regime has agreed in principle to attend, but the opposition has balked, saying it won't participate while "massacres are taking place."
In Geneva on Wednesday, U.N.-Arab League envoy Lakhdar Brahimi acknowledged a conference was unlikely before July, adding that "it is embarrassing for us that we are not capable of holding this conference already."
"The opposition has to complete a lot of work to get ready for this conference. ... Until the opposition is ready, all we can do is wait," he said.
"Sitting with a killer regime that is killing its people ... will not lead to what is required and is a waste of time," said Abdelbaset Sieda, a senior member of the main Western-backed Syrian National Coalition group.
In a rare statement read on state TV, the Syrian military triumphantly declared that it had "cleansed" Qusair of rebels. It said the town's capture was a "clear message to all those participating in the aggression against Syria" that the tide is turning in the war.
Images broadcast by media embedded with Syrian troops showed a deserted town with heavily damaged buildings and the Syrian flag flying over a clock tower in its main square. Syrian soldiers celebrated atop rubble, waving Syrian flags and chanting pro-regime slogans.
Regime forces leveled row after row of buildings in recent days to deny the rebels cover, and TV footage showed the town's church pockmarked with bullets and its mosque's dome damaged. About a dozen corpses could be seen on the ground.
Outnumbered and outgunned, rebel fighters held out for weeks after the regime launched its assault on Qusair on May 19. They inflicted heavier than expected casualties on the Hezbollah forces who joined the battle, forcing the group to acknowledge its involvement as dozens of its fighters were brought home for burial.
But the rebels were running short of ammunition, and they finally withdrew from the town after an intense bombardment overnight. Fighting continued Wednesday in areas on the northern edge of town where the rebels retreated.
During the barrage, doctors in the town managed to evacuate some 300 seriously wounded civilians who had been trapped during the siege. Convoys ferried them to the nearby village of al-Buweida, said Dr. Kazem Alzein, who helped coordinate the effort, speaking on Skype.
"We had to. They were destroying the neighborhood," he said.
Many residents fled to neighboring Lebanon's mainly Sunni border town of Arsal.
"There isn't a house that was not hit by at least one shell in Dabaa or Qusair. There are also villages that were totally destroyed," said a man who identified himself by his nickname, Abu Alaa, fearing retribution. He said he walked about 20 miles (35 kilometers) to safety in Lebanon on Tuesday.
Qusair formerly home to some 40,000 people was key for both sides. It lies on a land corridor linking two Assad strongholds, the capital of Damascus and an area along the Mediterranean coast that is the heartland of his minority Alawite sect.
For the rebels, Qusair was a crucial conduit for weapons, fighters and supplies smuggled into Syria from Lebanon.
The fall of Qusair provides the best evidence to date that the growing participation of Hezbollah fighters alongside Assad's troops is a potential game changer in the more than 2-year-old conflict that has left more than 70,000 people dead.
It remains to be seen whether this will prod the West to arm the rebels, who are no match for Hezbollah's military power and the regime's aerial superiority. A European arms embargo expired last week, freeing up individual nations to arm the rebels unilaterally.
The war already risks spilling over into Lebanon, where pro and anti-Syrian groups have battled each other in recent days and Syrian rebels have retaliated against Shiite towns near the Syrian border.
Qusair's fall risks drawing further revenge attacks.
"The repercussions will be on the Lebanese territories," Bassam al-Dada, an official in the rebels' Free Syrian Army, told The Associated Press.
George Sabra, acting head of the main Western-backed Syrian National Coalition, said the battle for Qusair was just "one round" in a larger fight to "liberate Syria."
At a news conference in Istanbul, he slammed Iran and Hezbollah's "murderous" involvement and warned Hezbollah's role in Syria will deepen the Sunni-Shiite rift in the region.
In a sign of the growing fears of a regional spillover, Jordanian officials on Wednesday said the U.S. will send anti-missile batteries and fighter jets to Jordan at the kingdom's request to boost defense capabilities in the face of an attack from Syria.
The equipment is being sent as part of preparations for an annual joint military exercise, but the officials said some would be kept in the country amid fears that the civil war raging in Syria will spill over the border.
A day earlier France and Britain made back-to-back announcements that the nerve gas sarin was used in Syria's conflict. A U.N. probe, also released Tuesday, said it had "reasonable grounds" to suspect small-scale use of toxic chemicals in at least four attacks in March and April.
The statements which included a confirmed case of the Syrian regime using sarin leave many questions unanswered, however, because the probes were mostly carried out from outside Syria, from samples collected by doctors and journalists.
Karam reported from Beirut. Associated Press writer Sarah El Deeb contributed to this report.