Fears that the Syrian regime could resort to using its arsenal of chemical weapons against rebels are well-founded, Western officials say, but experts at a security forum said Damascus is unlikely to unleash these arms.

"We are extremely concerned about the stockpiles of chemical and biological weapons, and we are also concerned about evidence during the last couple of weeks that the regime could use them," British Foreign Secretary William Hague told reporters on Saturday at the Manama regional security forum.

His views were echoed by US Republic Senator John McCain who said it was a "tough" question facing the global community.

The United States and the Arab world could "face a very, very tough decision, that is whether we should do anything, and if so, what, because of this threat that is now posed by (President) Bashar al-Assad assembling these weapons of mass destruction," McCain told participants at the gathering.

The international community has raised alarms over Damascus' stockpiles of such weapons, with UN chief Ban Ki-moon saying their use would constitute an "outrageous crime" and Washington warning that their deployment was a red line.

Washington fears battlefield advances by the rebels could prompt the Assad regime to use them, or that stocks could fall into the hands of groups hostile to the United States and its allies.

Turkey, which has witnessed a sharp deterioration in its ties with Syria, harbours similar fears.

Turkey's deputy foreign minister Naci Koru, who also took part in the Manama forum, said the Syrian "regime has lost its rationale together with its legitimacy."

"We all know very well the chemical and biological weapons capability of the regime."

Damascus ihas nsisted it would never deploy these arms in the 21-month conflict, that has killed more than 42,000 people according to activists.

On Saturday, Damascus even warned that the rebels could use such weapons.

"Terrorist groups may resort to using chemical weapons against the Syrian people... after having gained control of a toxic chlorine factory" east of Aleppo, the foreign ministry said.

The ministry was believed to be referring to the Syrian-Saudi Chemicals Company (SYSACCO) factory near Safira, which was taken over last week by militants from the jihadist Al-Nusra Front.

But some experts attending the Manama forum downplayed the fears and insisted that these weapons were not a game changer on the ground, with one analyst even saying they had limited operational utility.

"The regime is preparing the terrain for the use" of chemical weapons "but it does not mean they will use it," said Dina Esfandiary, a Non-Proliferation and Disarmament Programme researcher at London's International Institute for Strategic Studies (IISS).

Not much is known about Syria's chemical weapons programme, launched in the 1970s with support from Egypt and then from the former Soviet Union.

Esfandiary said Syria had a "long standing" and the "biggest" chemical programme in the Middle East with chemical agents "weaponised into missiles, artillery shells and chemical bombs."

She said the programme was set up to "offset Israel's nuclear programme" and that Damascus has a "few production facilities scattered around and a couple of storage facilities."

"My belief is if the international community does a good job by drawing a red line, they will not use it," she said, adding that important information on the programme was collected from officers who defected, including Adnan Silou, the former head of the programme itself.

Carnegie Middle East Centre analyst Yezid Sayigh argued that the Syrian regime, pushed into the corner, was using the presence of chemical weapons as a scarecrow against the international community.

"The primary use is for sending a deterrent message to the outside world... They are primarily a psychological weapon of terror," he said.

"The real utility in operational use in a war is very limited. It cannot be used in an area where there is a mix of friendly troops and hostile population or mixed population."