Syria's huge array of chemical weapons could fall into the hands of militants if President Bashar Assad was toppled, with "catastrophic" consequences, according to a report by senior British lawmakers published on Wednesday.

Britain's foreign intelligence services had no doubt Syria owned "vast stockpiles" of such weapons, including mustard gas, sarin, ricin and VX, the deadliest nerve agent, parliament's Intelligence and Security Committee (ISC) said in its report.

"There has to be a significant risk that some of the country's chemical weapons stockpile could fall into the hands of those with links to terrorism, in Syria or elsewhere in the region - if this happens, the consequences could be catastrophic," the committee said.

The conflict in Syria has killed 100,000 people, driven a 1.7 million more abroad as refugees and left swathes of urban Syria in ruins, although neither the violence nor economic collapse has truly shaken Assad's power base.

Nonetheless there was a risk of "a highly worrying proliferation around the time of regime fall," the head of Britain's Secret Intelligence Service told the committee.

Both forces loyal to Assad and rebels involved in the two-year uprising against the president have been accused of using chemical weapons during fierce fighting.

Syria is one of seven countries not to have joined a 1997 convention banning chemical weapons.

Last month, the United States said Assad's forces had used the nerve agent sarin on a small scale multiple times against opposition fighters, an assessment with which the British government said it agreed.

On Tuesday, Russian's UN envoy reported that Russian scientific analysis had indicated that Syrian rebels had also used sarin in an attack on the city of Aleppo in March. On Wednesday, the opposition Syrian National Coalition denied this report.

The committee said the SIS had told them that "the most worrying point about our intelligence on Syria's attitude to chemical weapons is how low a threshold they have for its use."

The report also said that Britain's spy chiefs believed al-Qaida groups and individual militants who have gained expertise and experience in Syria posed the biggest emerging  threat to the West.

"Large numbers of radicalized individuals have been attracted to the country, including significant numbers from the UK and Europe," it said.

Last week, Britain's top counter-terrorism official said the conflict in Syria had brought large numbers of al-Qaida fighters close to Europe for the first time.

Meanwhile, a Western diplomat said senior UN officials would head to Damascus soon to discuss ways of breaking the deadlock on access for a U.N. chemical weapons investigation team that has so far been unable to visit Syria.
So far, chief UN chemical weapons inspector Ake Sellstrom's team has not traveled to Syria because of diplomatic wrangling over the scope of access he would have there.

UN Secretary-General Ban Ki-moon wants Sellstrom to have unfettered access to investigate all credible alleged chemical attacks while Assad's government wants the UN experts to confine their investigation to the March 19 incident. That disagreement has caused a deadlock in talks between the United Nations and Syria on access for the inspection team.

Syrian UN Ambassador Bashar Ja'afari on Monday said his government has invited Sellstrom and UN disarmament chief Angela Kane to Damascus to discuss allegations of banned arms use in Syria's two-year civil war but suggested it would not compromise on access.

The senior Western diplomat said Sellstrom and Kane were expected to accept the invitation and travel to Damascus soon to discuss ways of breaking the deadlock.