'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.
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.
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.Continue reading the main story 'Present evidence'
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.
Models for possible intervention
- Iraq 1991: US-led global military coalition, anchored in international law; explicit mandate from UN Security Council to evict Iraqi forces from Kuwait
- Balkans 1990s: US arms supplied to anti-Serb resistance in Croatia and Bosnia in defiance of UN-mandated embargo; later US-led air campaign against Serb paramilitaries. In 1999, US jets provided bulk of 38,000 Nato sorties against Serbia to prevent massacres in Kosovo - legally controversial with UN Security Council resolutions linked to "enforcement measures"
- Somalia 1992-93: UN Security Council authorised creation of international force with aim of facilitating humanitarian supplies as Somali state failed. Gradual US military involvement without clear objective culminated in Black Hawk Down disaster in 1993. US troops pulled out
- Libya 2011: France and UK sought UN Security Council authorisation for humanitarian operation in Benghazi in 2011. Russia and China abstained but did not veto resolution. Air offensive continued until fall of Gaddafi
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.
|Country||Forces available for Syria strike|
Four destroyers - USS Gravely, USS Ramage, USS Barry and USS Mahan - in the eastern Mediterranean, equipped with cruise missiles Cruise missiles could also be launched from submarines in the region
Airbases at Incirlik and Izmir in Turkey could be used to carry out strikes
Two aircraft carriers - USS Nimitz and USS Harry S Truman
Cruise missiles could be launched from a British Trafalgar class submarine
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
Airbase in Cyprus could also be used
Aircraft carrier Charles de Gaulle is currently in Toulon.
Raffale and Mirage aircraft can also operate from Al-Dhahra airbase in the UAE