An early morning assault today by the Syrian army on the key western Syrian city of Qusair forced the withdrawal of rebel fighters after almost three weeks of intense and bloody fighting.

The victory by Syrian soldiers - backed by fighters from the Lebanese militant group Hezbollah - marks a significant turn that highlights a renewed offensive push by the Syrian army, deals a blow to the flow of weapons and fighters to the rebel side and raises the specter of increased spillover violence in Lebanon.

Syrian state media trumpeted the "full defeat and collapse" of the rebel side which "restored security and stability" to Qusair, held for a year by the rebels.

A statement on the state-run SANA news agency website said the army had seized and destroyed weapons "from the armed terrorist groups" while also killing a large number of the rebels. Photos were posted of soldiers parading through the town with Syrian flags, as well as heavy weapons the army found.

The battle for Qusair also marked the deepest foray into Syria's two-year civil war by Hezbollah, the first time in its 30-year history that it had gone into another country to fight. In Qusair, Hezbollah fighters backed the Syrian forces and is believed to have lost scores of fighters in the fighting. In recent weeks, Hezbollah leader Sayyed Hassan Nasrallah has vowed that his group will continue supporting the forces of President Bashar al-Assad, "and I promise you victory," he said.

For the Shiite group Hezbollah, this is viewed as an existential fight; Syria is led by Alawites - an offshoot of Shiite Islam - and is Hezbollah's biggest Arab backer and the gateway for support and weapons from Shiite Iran. The vast majority of the rebel fighters are Sunni Muslims, mostly armed and funded by the fundamentalist Sunni countries Saudi Arabia and Qatar.

"We are now in a totally new phase that began weeks ago," Nasrallah said in a televised speech two weeks ago. "This new phase aims at fortifying the resistance [Hezbollah] and its protecting its backbone…"

"Syria is the backbone and supporter of the resistance," he added. "The resistance cannot stand by idly by when it's backbone is exposed."

The rebels fighting in Qusair conceded that they had pulled out "in face of this huge arsenal and lack supplies and the blatant intervention of Hezbollah."

"Dozens of fighters stayed behind and ensured the withdrawal of their comrades along with the civilians," they said in a statement to the Reuters news agency.

Qusair sits near the border with Lebanon and has seen a steady supply of fighters and arms arriving from northern Lebanon for the rebel side. By taking Qusair, the regime helps secure the supply route from the predominantly Alawite province of Latakia on the coast, down through the city of Homs to the capital Damascus. Qusair has also been used in the past to funnel arms to Hezbollah from Syria and Iran.

"Whoever controls Qusair controls the center of the country, and whoever controls the center of the country controls all of Syria," said Syrian Brigadier General Yahya Suleiman to pro-Assad Al Mayadeen television.

Hezbollah's vocal involvement in Syria has resulted in a spike of cross-border sectarian attacks between Lebanon and Syria. The northern Lebanese city of Tripoli has also seen renewed deadly fighting between Sunnis and Alawites. The head of the rebel Free Syrian Army, General Salim Idriss, told the BBC today that Hezbollah's involvement in Syria could lead to rebels taking the fight to Hezbollah inside Lebanon.

"The Syrian opposition will have to take necessary measures if the Lebanese authorities fail to end Hezbollah's interference in Syria battles," he warned.