DAMASCUS (AFP) - Syria's army has overrun the strategic region of Qusayr after a blistering offensive against rebel fighters, state television said on Wednesday, marking a major victory for regime forces in the 26-month conflict.

The battlefield success comes as officials from Russia, the United States and the United Nations gathered in Geneva on Wednesday to work on a peace conference for Syria in the wake of fresh allegations that the Damascus regime has used chemical weapons.

"The Syrian army totally controls the Qusayr region in Homs province after killing a large number of terrorists and capturing others," the television report said, using the regime's terminology for the rebels.

President Bashar al-Assad's forces backed reportedly by thousands of fighters from Lebanon's Hezbollah movement launched an offensive to retake Qusayr, a key region on the Lebanon border, on May 19.

Rebels fighting to retain control of Qusayr town were later joined by hundreds of reinforcements from Lebanon, most close to the Muslim Brotherhood, leading to pitched battles in which civilians, many of them wounded, became trapped.

Doctors had appealed for the Red Cross to be allowed in to treat the wounded but Syrian officials said this would only be permitted once the rebels had been defeated.

Civilians who had managed to flee Qusayr described it as "a ghost town, heavily damaged and filled with the sound of bombs," the UN refugee agency UNHCR said Tuesday.

Those who had escaped were mainly women and children because men risked being killed at checkpoints, said spokeswoman Melissa Fleming.

Control of Qusayr was vital for the rebels as it is their principal transit point for weapons and fighters from across the border in Lebanon.

It is also strategic for the regime because it is located on the road linking Damascus with the coast, its rear base.

On the diplomatic front, the officials meeting in Geneva hope to hammer out terms to get President Assad's camp and the rebels to negotiate directly for the first time.

The meeting comes a day after France said it had proof that Assad's regime had used the deadly nerve agent sarin gas in Syria's civil war.

Earlier, a UN report said there were "reasonable grounds" to believe that both sides in Syria had used chemical weapons.

Washington however said it needed more evidence before concluding that sarin had been used.

French Foreign Minister Laurent Fabius told France 2 television: "We have no doubt that the gas is being used. The conclusion of the laboratory is clear: there is sarin gas."

Experts had analysed samples brought back from Syria, he said. They had sent their results to United Nations experts investigating the issue.

Earlier, Mr Fabius had said in a statement: "France is now certain that sarin gas has been used in Syria several times and in a localised fashion."

It did not say who had used the gas, but in a televised interview later, Mr Fabius said that in at least one case, there was "no doubt that it was the regime and its accomplices".

"A line has been indisputably breached...," said Mr Fabius. "We must react, but at the same time we must not block an eventual peace conference," he added.

Washington and Moscow are trying to get the Syrian regime and the opposition together to negotiate a way out of the war, which has cost more than 94,000 lives in 26 months of fighting.

But a tentative date for the conference, initially planned for early June, has slipped back into July amid wrangling over the exact guest list and agenda.

The White House reacted cautiously to the latest allegations of chemical warfare: they wanted to know more before concluding such weapons had been used.

"We need to expand the evidence we have ... before we make any decision," said spokesman Jay Carney. President Barack Obama has said that the use of the deadly nerve agent would be a "game-changer" for Washington.

In Geneva on Tuesday, the Commission of Inquiry on Syria stated: "There are reasonable grounds to believe that chemical agents have been used as weapons."

It was the first time it had made such an allegation since starting work in 2001.

Most of the incidents concerned their use by government forces, it said in its report to the UN Human Rights Council.

"The conflict in Syria has reached new levels of brutality." it added.

Government forces and their militia had committed "murder, torture, rape... and other inhumane acts," against civilians.

Rebel groups had also committed war crimes including murder, torture, hostage-taking and pillage, but on a lesser scale, it said.

It called on Syria to let UN investigators in to check for the possible use of chemical weapons, something the regime has so far refused.