'Arm Syrian rebels', says Blair.

Former UK Prime Minister Tony Blair has warned of potentially "catastrophic consequences" if the west fails to intervene in the Syrian civil war.

Speaking to the Times, Blair urged Britain to help the US arm rebel forces and advocated imposing a no-fly zone over Syria.

"I feel very strongly that we are in danger of a failure, with catastrophic consequences," he said.

"This is no longer a civil war between factions within Syria. We should be taking a more interventionist line."

He warned that the conflict was spiralling out of control, partly as a result of the intervention of Iran.

"You've got the intervention of Hezbollah, at the instigation of Iran...You've got to create the circumstances in which Assad is not able to change the balance of power within the struggle by the use of outside forces."

President Obama this week claimed that there was evidence that chemical weapons had been used by forces loyal to Syrian president Bassar al Assad and that the US would arm rebel fighters and was considering imposing a no-fly-zone over the country.

"The other big change is the use of chemical weapons," said Blair, who has worked as a peace envoy in the Middle East since resigning as prime minister in 2007.

"Once you allow that to happen - and this will be the first time it has happened since Saddam used them in the 1980s - you run the risk of it then becoming an acceptable form of warfare, for both sides."

"You don't have to send in troops, but the international community should think about installing no-fly zones," he added.

Russia along with Iran has allied itself with the Assad regime, and it is thought the Russians may use their UN Security Council veto to block an international resolution on armed intervention in Syria from the west.

UK Prime Minister David Cameron is scheduled to meet Russia's President Vladimir Putin on Sunday 16 June, ahead of the G8 summit, where Syria is expected to be top of the agenda.

Addressing parliament this week, Cameron also claimed that chemical weapons have been used in the conflict, but has stopped short of committing the UK to intervention.

Critics claim that weapons shipments from Europe and the US could be turned against the west by Al-Qaida elements in the Syrian rebel forces.

The civil war, which broke out in 2011, is believed so far to have claimed over 93,000 lives.