The British government has published internal legal advice it says shows it is legally entitled to take military action against Syria even if the UN Security Council ruled against it.

The news comes after both Britain and Russia announced they were deploying military resources to strategic areas as tensions grow in the Syrian crisis.

The British government is insisting there should be a military response to the Syrian government's use of chemical weapons, but getting political and popular backing for intervention will not be easy.

British MPs were recalled early from their summer holidays in the hope of gaining parliamentary support for military action against Syrian president Bashar al-Assad.

The leader of Britain's opposition Labour Party, Ed Miliband, says parliament should not have to decide whether to approve military intervention in Syria on what he calls an "artificial timetable".

Key points: Russia sends warships to the Mediterranean, but says it is nothing out of the ordinary.

Russia sends warships to the Mediterranean, but says it is nothing out of the ordinary. Britain sends six Typhoon jets to base in Cyprus.

Britain sends six Typhoon jets to base in Cyprus. France says strike plans against Assad regime "difficult to develop".

France says strike plans against Assad regime "difficult to develop". UN team investigating chemical weapons use to leave Syria on Saturday

"This is a very grave decision to take military action that the House of Commons is deciding," he said on Thursday.

"And I didn't think that that decision should be made on an artificial timetable when the House of Commons wouldn't have even seen the evidence today from the United Nations weapons inspectors.

"I'm determined we learn the lessons of the past, including Iraq."

Russia moves warships into Mediterranean

Russia, who has stood staunchly behind the Syrian regime during its two-year civil war, is sending an anti-submarine ship and a missile cruiser to the Mediterranean

Russia has warned Western powers about military strikes against Assad's forces and has previously stymied efforts to find a political solution through the UN.

Russia's Interfax news agency quoted a source in the armed forces' general staff as saying the warships would arrive in the coming days because of the "well-known situation" - a clear reference to the conflict in Syria.

The navy later denied the deployment was linked to events in Syria and said it was part of a long-planned rotation of its ships in the Mediterranean.

It did not say what kind of vessels, or how many, were on their way to the region. However, the initial Interfax report had made clear that the aim was to beef up the navy's presence and not to replace the ships in the Mediterranean.

UK downplays deployment of Typhoon fighter jets

Meanwhile, Britain has deployed six RAF Typhoon jets to its Akrotiri base in Cyprus in a move it says is to protect British interests in the region.

"This is purely a prudent and precautionary measure to ensure the protection of UK interests and the defence of our sovereign base areas at a time of heightened tension in the wider region," the ministry of defence said.

Sorry, this video has expired Lama Fakih from Human Rights Watch says military action in Syria must adhere to international law.

It also added the jets would not take part in any direct military action against Syria.

The strategic movement of military assets comes as Western powers mull a response to the alleged use of chemical weapons by Assad forces.

Britain has been pushing for permanent members of the UN Security Council to adopt a resolution which would have authorised measures to protect civilians in Syria.

But Russia has refused to agree to the resolution and a diplomatic stalemate continues.

Prime Minister Kevin Rudd earlier joined the US and Britain in condemning the use of chemical weapon in Syria, labelling it "an offence against humanity and arguably is a crime against humanity".

Plans on Syria strike 'difficult to develop'

Western plans for a strike against the Assad regime are "difficult to develop", a French government spokeswoman said on Thursday.

"The international community must find a riposte that is adapted to the situation," Najat Vallaud-Belkacem said on France 2 television.

She said it was "necessary to obtain the adhesion of several allies and partners at the heart of the UN Security Council, which we are trying to do" but added that "states like Russia and China pose a certain number of problems."

The aim of military action "will not simply be to punish the Syrian regime and prevent it from carrying out a new attack of this type ... but also to seek a way out of this crisis."

"It's extremely important for the international community, if it intervenes, to do so in a manner that the country can recover."

Her comments came after US president Barack Obama, who had warned that the Assad regime would be crossing a "red line" if it used chemical weapons, said he had not yet made a decision on retaliatory strikes.

Syria denies using chemical weapons and has blamed the opposition for the August 21 attack on a Damascus suburb that killed hundreds of civilians.

Mr Assad said overnight that Syria will not shy away from a threat to its sovereignty.

"Syria will defend itself in the face of any aggression, and threats will only increase its commitment to its principles and its independence," he said, according to state television.

US has 'no choice' in releasing intelligence

Michael Singh, the managing director of the Washington Institute, says if the US has intelligence that proves the Assad regime is behind the chemical weapons attack, it must make it public.

"I think they'll have no choice," he told the ABC's Lateline program.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 13 minutes 44 seconds 13 m Michael Singh, the managing director of the Washington Institute, says the US needs to think about the "end game" in Syria.

"I think that the US Government, as well as the British government and others governments, will have to publicly share what information has led them to conclude that the regime is responsible for these attacks in Syria.

"Because of course the entire justification for military action is based upon punishing and deterring chemical weapons attacks."

He said as US officials begin to map possible targets - including air bases, missile launchers and artillery batteries - they need to have a clear objective in mind.

"I would say that simply punishment really can't be enough," he said.

"If you're going to go in and use military force, you need to think: what's the sort of end game here?"

UN inspectors to pull out at the weekend

The UN team trying to determine whether chemical weapons had been used in the area will continue their investigations until Friday and plan to leave Syria by Saturday morning.

UN secretary-general Ban Ki-moon, speaking in Vienna, said he had spoken to Mr Obama on Wednesday about the situation in Syria, discussing how "we can expedite the process of investigation."

"I have also expressed [my] sincere wish that this investigation team should be allowed to continue their work as mandated by the member states," Mr Ban told reporters on Thursday.

"I told him that we will ... share information and our analysis of samples and evidence with members of the Security Council and United Nations members in general," he said.

UN chemical weapons experts on Thursday began their third day of investigations into the apparent poison gas attack.

ABC/wires