27 August 2013 Last updated at 22:24 ET 
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Professor Michael Clarke: ''There is absolutely no quick military solution to this crisis''
David Cameron has discussed Syria with Barack Obama by telephone as Britain and the US consider intervention.
The UK's response will be discussed at a National Security Council (NSC) meeting later, while the US has said its forces are "ready to go".
The prime minister said on Tuesday the world could not "stand idly by" as the Syrian government used chemical weapons against its own people.
But Syria's foreign minister denied his government had used chemical weapons.
Mr Cameron said Britain could intervene to stop Syria using such weapons, following a suspected attack last week which is being investigated by the United Nations.
MPs have been recalled from their summer break early and Mr Cameron said a motion would be put to them for a vote on Thursday.
He did not give details of the motion, but said any intervention in Syria would be "legal" and "proportionate".
The phone call between Mr Obama and Mr Cameron was the second since the alleged chemical attack, but the prime minister is not expected to put forward their preferred course of action until after the NSC meeting.
Continue reading the main story Analysis
Caroline Wyatt Defence correspondent, BBC News
The aim is not regime change, according to Downing Street and the White House - but what they term "limited action" to show Syria and others that the use of chemical weapons will be punished.
With just under two million Syrian refugees already, one worry is that any military intervention could create even more. And targets will have to be chosen carefully ahead of any military action - which could, according to some sources, happen within days.
The west will have to be careful to avoid any civilian casualties.
And there is a real risk of retaliation - whether by the Syrian regime, or even one of its supporters. They include Russia and China - and Iran could also react. Retaliation could hit allies in the region or places such as Cyprus.
And another risk: that a military strike could help the opposition, who include elements linked to Al Qaeda - the very people the west doesn't want to have any access to chemical weapons.
Foreign Secretary William Hague echoed Mr Cameron's comments.
Letting such an attack go unchallenged would "make further chemical attacks in Syria much more likely, and also increase the risk that these weapons could fall into the wrong hands in the future," he wrote in the Daily Telegraph.
BBC defence correspondent Caroline Wyatt said any military strikes would probably focus on command centres believed to be involved in the use of chemical weapons.
She said cruise missiles could be launched from US ships in the Gulf or the Mediterranean, or Royal Navy vessels including submarine HMS Tireless.
Professor Michael Clarke, of the Royal United Services Institute, said there was ''absolutely no quick military solution'' to the crisis.
"Any involvement in striking Syria, in effect becomes an involvement in the civil war… we are siding then, with the opposition,'' he added.
Chemical claims The Syrian authorities have blamed opposition fighters for last Wednesday's attack near Damascus, which reportedly killed more than 300 people.
Foreign Minister Walim Moualem said the incident was being used as a pretext for intervention by the US and its allies.
But Mr Hague said in the Telegraph: "To argue that the Syrian opposition carried out this attack is to suggest that they attacked their own supporters in an area they already controlled using weapons systems they do not possess."
And US Vice-president Joe Biden said there was "no doubt" the Syrian government was responsible.
Vessels such as HMS Tireless could launch strikes on Syria, says the BBC's Caroline Wyatt
Nick Clegg, whose Liberal Democrats opposed the invasion of Iraq in 2003, said Britain would set a "very dangerous precedent" if no action were taken after a chemical attack on civilians - though he said there would be no "boots-on-the-ground" invasion.
Labour leader Ed Miliband said his party would "consider supporting international action", but only if it was legal and "specifically limited to deterring the future use of chemical weapons".
Conservative MP John Barron said acting without UN approval "risks reintroducing the law of the jungle when it comes to international law".
The Archbishop of Canterbury, Justin Welby, warned MPs not to rush any decision as the facts were still unclear and the consequences of military action across the Muslim world were unpredictable.
The Stop the War Coalition called on the British public to oppose what it called "another disastrous military intervention".
Jack Straw, who was foreign secretary at the time of the Iraq invasion, said international law had "broadened" since then to make military intervention without UN backing possible in humanitarian situations.
But he said any action would need legal approval from Britain's attorney general.
General Lord Dannatt - head of the British Army until 2009 - said military action without UN backing would be "wrong", and said Mr Cameron must "convince the British people that there is a clear case for intervention".
Russia has warned that any foreign involvement in Syria without a UN mandate - over which it holds the power of veto - would be "a grave violation of international law".
| Country | Forces available for Syria strike |
|---|
| US | Four destroyers - USS Gravely, USS Ramage, USS Barry and USS Mahan - are in the eastern Mediterranean, equipped with cruise missiles. Cruise missiles could also be launched from submarines. Airbases at Incirlik and Izmir in Turkey, and in Jordan, could be used to carry out strikes. Two aircraft carriers - USS Nimitz and USS Harry S Truman - are in the wider region. |
| UK | Cruise missiles could be launched from a British Trafalgar class submarine. HMS Tireless was reportedly sighted in Gibraltar at the weekend. The Royal Navy's response force task group - which includes helicopter carrier HMS Illustrious and frigates HMS Montrose and HMS Westminster - is in the region on a previously-scheduled deployment. RAF Akrotiri airbase in Cyprus could also be used. |
| France | Aircraft carrier Charles de Gaulle is currently in Toulon in the western Mediterranean. Raffale and Mirage aircraft can also operate from Al-Dhahra airbase in the UAE. |
