Prime Minister David Cameron said the world could “not stand idly by” after seeing “appalling scenes of death and suffering” caused by suspected chemical weapons attacks.
The crisis follows last Wednesday’s suspected chemical attack near the Syrian capital, Damascus, which reportedly killed more than 300 people.
Senators John McCain and Lindsey Graham are insisting that President Obama act now against the Syrian government.
US Vice President Joe Biden said there was “no doubt who was responsible for this heinous use of chemical weapons in Syria: The Syrian regime”.
In a speech to a veterans’ group in Houston, Mr Biden said that “those who use chemical weapons against defenceless men, women, and children… must be held accountable”.
French President Francois Hollande said France was “ready to punish” whoever was behind the attack, and had decided to increase military support for Syria’s main opposition.
The US, UK and France will now have the larger task of building as wide a coalition as possible to support limited military action.
Meanwhile the Arab League said it held Syrian President Bashar al-Assad responsible for the attacks and called for UN action. Syrian opposition sources have said they have been told to expect a Western intervention in the conflict imminently.
“There is no precise timing… but one can speak of an imminent international intervention against the regime. It’s a question of days and not weeks,” AFP news agency quoted Syrian National Coalition official Ahmad Ramadan as saying.
Russia and China, allies of the Syrian government, have stepped up their warnings against military intervention, with Moscow saying any such action would have “catastrophic consequences” for the region.
The U.S. Navy has now moved warships into the Mediterranean and has readied Cruise Missiles for a possible strike against government forces.
The UN says more than 100,000 people have been killed since the uprising against President Assad began more than two years ago. The conflict has produced more than 1.7 million registered refugees. How many more after this strike?
BBC Arabic, Damascus
A good number of Syrians, in particular those supporting the regime, believe the visit of the UN chemical weapons investigation team is nothing but a move to justify a military attack on Syria. The opposition, however, thinks that these visits will lead to some evidence being unearthed, proving that chemical weapons have been used against civilians by the Syrian regime.
Above all, fear and discomfort are palpable among those living in the capital. People are haunted by the possibility of a Western military strike on Syria, discussion of which is dominating the headlines of satellite channels.
“I don’t want Syria to become another Iraq… Enough bloodshed,” cried one Syrian woman.
“We, and thousands like us across Syria, will face any country that tries to attack us,” threatened a young man, pointing at his weapon, which he uses to protect his neighbourhood. “These are Syria’s problems and it is up to us, Syrians, to solve them.”