'No doubt' Syria used chemical arms, says US Vice-President Joe Biden
US Vice-President Joe Biden has said there is "no doubt" that the Syrian government has used chemical weapons and that it must be held accountable.
The US has said its military is ready to launch strikes should President Barack Obama order an attack, and allies say they too are ready to act.
The Syrian government has strongly denied claims it used chemical weapons.
UN weapons inspectors are set to return to the site of last week's suspected attack near Damascus on Wednesday.
Their evidence-gathering visit was delayed by a day after they were fired on.
The US says it will release its own intelligence report into the incident at Ghouta, a suburb of the capital, in the coming days.
More than 300 people reportedly died there.
President Obama is said to have made at least 88 calls to foreign leaders since Wednesday's suspected attack, and spoke to UK Prime Minister David Cameron for the second time on Tuesday.
Mr Cameron said the world could "not stand idly by", and French President Francois Hollande said France was "ready to punish" whoever was behind the attack.
The UK's National Security Council is due to convene on Wednesday to discuss possible responses, while Parliament is to be recalled on Thursday.
Mr Cameron's office said the UK government had not yet taken a decision on the specific nature of a response, but that it would be "legal and specific to the chemical weapons attack".
On Wednesday, Russian Foreign Minister Sergei Lavrov warned that "attempts at a military solution will lead only to the further destabilisation" in Syria and the region.
Mr Lavrov emphasised the need for a political solution in a phone call to the joint UN-Arab League envoy to Syria, Lakhdar Brahimi, the foreign ministry in Moscow said.
Russia, China and Iran have previously warned against launching an attack on the war-ravaged country, where more than 100,000 people are thought to have died in two years of fighting.
Stocks have fallen on global markets and oil prices have shot up amid growing concern about an impending attack.
No 'regime change'
The US has not yet released its intelligence report into the alleged chemical attack, but US officials now say they are certain the Syrian government was behind the incident.
Mr Biden is the most senior member of the Obama administration to blame the Syrian government for the attack.
In a speech to a veterans' group in Houston, he said there was "no doubt who was responsible for this heinous use of chemical weapons in Syria: the Syrian regime".
He said that "those who use chemical weapons against defenceless men, women, and children... must be held accountable".
White House spokesman Jay Carney earlier said it would be "fanciful" to think anyone else could be responsible - saying the Syrian regime remained in control of the country's chemical arsenal and used the type of rocket that carried the payload used last Wednesday.
But he insisted there were no plans for "regime change". Any military campaign is likely to be limited in scope, with missile strikes targeting military sites and no ground troops.
US Defence Secretary Chuck Hagel told the BBC on Tuesday the US military had "moved assets in place" so all options were available to the president.
BBC diplomatic correspondent James Robbins says the US, UK and France will now have the larger task of building as wide a coalition as possible to support limited military 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.
"There have been meetings between the Coalition, the [rebel] Free Syrian Army and allied countries during which possible targets have been discussed."
Meanwhile the Arab League said it held Syrian President Bashar al-Assad responsible for the attacks and called for UN action.
But both Russia and China are permanent members of the UN Security Council and yield a veto on possible action.
Syrian Foreign Minister Walid Muallem demanded evidence that the government had used chemical weapons.
"I challenge all countries who [are] saying that Syria used chemical weapons to present their evidence to their public opinion," he told the BBC.
He told a news conference that faced with the choice of surrendering or defending itself, Damascus would fight.
Meanwhile, Russian Deputy Prime Minister Dmitry Rogozin tweeted that: "The West behaves towards the Islamic world like a monkey with a grenade."
China's state news agency recalled that intelligence used to justify the 2003 invasion of Iraq turned out to be flawed, while Syria's neighbour and ally Iran said any strike would threaten the stability of the region.