Any action against Syria would be aimed at "degrading" the regime's alleged chemical weapons' capability, British Prime Minister David Cameron said Tuesday.

After announcing that the British parliament would be recalled on Thursday to debate possible action, Cameron described the use of chemical weapons as "morally indefensible".

But any military action against Syria would have to be proportionate and legal, the prime minister said.

Cameron said no decisions had been taken but Britain and its allies had to consider whether targeted military action was required to "deter and degrade the future use of chemical weapons".

"This is not about wars in the Middle East; this is not even about the Syrian conflict. It's about the use of chemical weapons and making sure as a world we deter their use," he said.

"The question we need to ask is whether acting or not acting will make the use of chemical weapons more prevalent?"

Earlier, British Deputy Prime Minister Nick Clegg denied that the aim would be to topple the regime of President Bashar al-Assad.

"What we're considering is a serious response (to chemical weapons use)," he said.

"What we're not considering is regime change, trying to topple the Assad regime, trying to settle the civil war in Syria one way or another."

Foreign Minister William Hague later warned that diplomatic deadlock must not prevent action against Assad.

"All previous attempts to get the UN Security Council to act on Syria have been blocked, and we cannot allow diplomatic paralysis to be a shield for the perpetrators of these crimes," he wrote in the Daily Telegraph.

Britain, France and the United States have all said they believe the alleged gas attacks that killed hundreds of civilians near Damascus last week were launched by the regime.

Russia, also a veto-wielding member of the Security Council, has repeatedly warned Washington and its allies against military action in Syria.

A lack of action would undermine "decades of painstaking work to construct an international regime of rules and checks, overseen by the UN, to prevent the use of chemical weapons and to destroy stockpiles", Hague warned.

However, the minister accepted that taking action "will not be without risks" but argued "the risks of doing nothing are greater."

US Defence Secretary Chuck Hagel said US forces were ready to launch strikes against the Syrian regime once President Barack Obama gave the order.

In Paris, French President Francois Hollande said his country was "ready to punish" those behind the alleged attacks.

British lawmakers will debate Syria on Thursday.

In tense scenes a decade ago, the House of Commons gave its approval to the invasion of Iraq by a comfortable margin in March 2003 when Tony Blair was prime minister.

Two years ago, they gave overwhelming support -- 557 to 13 -- to airstrikes to assist the Libyan rebel forces.

It is the fourth time Cameron has recalled parliament during a recess -- previously a rarely-used step. The last time was for a day of tributes to former prime minister Margaret Thatcher, who died in April.