The Syrian army triumphantly announced Wednesday the capture of a strategic border town after a three-week gruelling battle, telling the nation it has "cleansed" Qusair of rebels and calling it "a message" to Syria's enemies everywhere.

The capture of the town, which lies close to the Lebanese border, solidifies some of the Syrian regime's recent gains on the ground that have shifted the balance of power in the Syrian civil war towards President Bashar al-Assad.

It comes just a day after France and Britain made back-to-back announcements that the nerve gas sarin was used in Syria's conflict. A UN 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 in Syria.

The statements — which included a confirmed case of the Syrian regime using sarin — leave many questions unanswered, however, because the probes were mostly completed outside Syria from samples collected by doctors and journalists.

In 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. Syrian troops, backed by scores of Lebanese Hezbollah fighters, launched a wide offensive on Qusair, near the border with Lebanon, on May 19.

A rare statement by the Syrian Armed Forces was read out Wednesday on state TV, saying the military had restored "peace and security" in Qusair, and that the town's capture was a "clear message to all those participating in the aggression against Syria."

The army said it cleared Qusair and surrounding villages in the country's west of "terrorists," the term the regime uses for rebels fighting to topple Assad's government and that a "large number [of rebels] have been killed.

Others surrendered while "the rest escaped" following a decisive push into the town late Tuesday, the statement said.

Hezbollah's involvement a potential game changer

Images broadcast by media embedded with Syrian troops showed a deserted Qusair, with heavily damaged buildings. Military bulldozers were removing rubble and clearing roads as armoured vehicles whizzed by.

The fall of Qusair provides the best evidence to date that the growing participation of militant Hezbollah fighters alongside Assad's troops is a potential game changer in the two-year-old conflict.

The Qusair battle also has laid bare Hezbollah's role in the Syrian conflict. The Shia group, which has been fighting alongside Assad's troops, initially tried to play down its involvement, but could no longer do so after dozens of its fighters were killed in Qusair and buried in large funerals in Lebanon.

Hezbollah's leader, Hassan Nasrallah, firmly linked the group's fate to the survival of the Syrian regime, raising the stakes not just in Syria, but also in Hezbollah's relations with rival groups in Lebanon.

The overt involvement of Hezbollah in the Qusair battles has further inflamed sectarian tensions in Syria and Lebanon, drawing retaliation from Syrian rebels on Lebanese Shia towns and villages near the border with Syria.

Qusair's fall risks drawing in more 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. "It is the beginning of the end for the group, even inside Lebanon," he added.

Qusair a conduit for weapons, fighters

Qusair's fall deals a huge blow to the opposition, which conceded the town's loss on Wednesday.

"The Assad regime and the Iranian militias supporting it have entered Qusair," a statement by the main Western-backed Syrian National Council said.

The overwhelmingly Sunni town has served as a conduit for shipments of weapons, fighters and supplies smuggled from Lebanon to the rebels inside Syria.

The Britain-based Syrian Observatory for Human Rights said Qusair came under intense overnight shelling, forcing the rebel fighters, short of ammunition, to withdraw. The Observatory said it fears for the fate of over 1,000 wounded.

Earlier, doctors in Qusair had said wounded civilians and fighters in need of critical medical attention have been trapped in the town, and pleaded for safe passage to transport them.

Both sides in Syria's conflict value Qusair, which lies along 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, who seized control of the town shortly after the uprising against Assad began in March 2011, holding Qusair meant protecting their supply line to Lebanon, just 10 kilometres away.

As the battle for Qusair intensified in the past week, rebels in the town called on fighters from all over Syria to come to their aid, and foreign fighters were suspected to be playing a large role in the city's defence.

'The town is empty'

Lebanon's Al-Mayadeen TV, which has reporters embedded with Syrian troops and was reporting live from Qusair, said there was no sign of fighting Wednesday. Hezbollah's TV channel, Al-Manar, showed pictures of seized weapons, and missiles in the town.

In the footage, the municipal building in the centre of Qusair and the town church appeared to be pockmarked from the fighting. A Syrian flag was raised above it in a show of government control.

A witness from the town, speaking to The Associated Press on condition of anonymity because he feared for his security, said the military was removing mines from around Qusair and clearing roads.

"The town is empty," the witness said over the telephone.

Syria is suspected of having one of the world's largest chemical weapons arsenals, including mustard and nerve gas, including sarin. In recent weeks, the regime and those trying to topple Assad have traded accusations of chemical weapons' use but offered no solid proof.

In the West, the lack of certainty about such allegations is linked to a high stakes political debate over whether the U.S. should get more involved in the Syrian conflict, including by arming rebel fighters. More than 70,000 people have been killed and several million displaced by the Syrian conflict since it erupted more than two years ago.