AMERICAN President Barack Obama is intending to use any military strike on Syria not just to punish the Assad regime for the Damascus chemical attack but also to "degrade" seriously its ability to take on the rebel forces, it has been claimed.

As the United Nations revealed the extent of Syria's "humanitarian calamity" with some two million people having now fled to neighbouring countries, Mr Obama expressed confidence he would win next week's key vote in the US Congress on military intervention.

While he received what appeared to be co-ordinated backing from Congressional leaders for a strike on Syria, the intentions of the rank and file members of Congress are difficult to predict, particularly as the American public is split down the middle on the issue.

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General Jack Keane, a former US Army vice-chief of staff, said he understood Mr Obama was planning a more substantial intervention in Syria than had previously been believed, with increased support for the opposition forces, including military training by US troops.

After speaking to Republican senators John McCain and Lindsey Graham, who were briefed by the President, General Keane said: "What he has told the two senators is that he also intends to assist the opposition forces, so he is going to degrade Assad's military capacity and he is going to assist and upgrade the opposition forces with training assistance."

At Westminster, No 10 repeatedly refused to rule out the possibility the Lib-Con Coalition could return to the Commons with a new motion on upgrading its support for the Syrian rebels. But a source insisted: "Arming the opposition is just not on the cards."

Prime Minister David Cameron said ruling out British military action did "not mean we do nothing on Syria", stressing the UK was the second largest donor of humanitarian aid.

In the Commons, Foreign Secret­ary William Hague sought to play down suggestions Mr Obama was planning a wider-than-expected intervention, saying: "I don't believe that to be the intention of the United States."

But one senior MP noted: "It seems increasingly clear Obama intends to use the cover of a strike on destroying Assad's chemical arsenal to degrade his entire military capability."

In Washington, the President stressed how the Assad regime's use of chemical weapons was a threat to America's national security as he began a charm offensive to bring Congress behind his Syria strike plan. He described it as "appropriate, proportional and limited" but would not involve US boots on the ground.

"This is not Iraq and this is not Afghanistan," Mr Obama declared, explaining any strikes would "degrade Assad's capabilities when it comes to chemical weapons" but also fit into a "broader strategy to make sure we can bring about over time the kind of strengthening of the opposition and the diplomatic and economic and political pressure required so that, ultimately, we have a transition that can bring peace and stability not only to Syria but to the region".

Other developments included:

l The Syrian opposition claimed a forensic scientist had defected to the rebel side bringing evidence of Assad forces' use of sarin gas

l Ban Ki-Moon, the UN Secretary General, said the use of chemical weapons in Syria would be a war crime

l Israel tested a US-backed missile system in the Mediterranean, saying it was long-planned, routine and had nothing to do with the Syrian crisis

l French President Francois Hollande said he would wait for the US Congress vote, insisting France would not strike alone.