President Barack Obama has said the credibility of the US, its Congress and the international community is on the line over their response to Syria’s alleged use of chemical weapons.
Speaking in Sweden, he said the world should stick to its own “red line” against the use of chemical weapons.
Mr Obama is trying to build support in the US for punitive military action against the Syrian government.
The US Congress will vote next week on whether to support his proposed action.
France – whose government has strongly backed the US plan for intervention – is holding an extraordinary debate in the National Assembly, though MPs will not vote on the matter.
Prime Minister Jean-Marc Ayrault stressed the need to take action together with France’s partners, saying: “Faced with barbarism, doing nothing is not an option.”
He said it was “France’s honour, France’s duty” to act and that President Francois Hollande was continuing efforts to bring together a coalition.
“What message would this send to other regimes, and I am thinking like you of Iran and North Korea? The message would be clear: You can continue,” he added.
Iran has rejected Western claims that it is seeking to build a nuclear weapon, while North Korea has a nuclear bomb but not yet the means to deliver it via a missile.
The UK parliament voted last month against military intervention in Syria.
Russia remains firmly opposed, with President Vladimir Putin warning on Wednesday that any strike without UN approval would be “an aggression”.
But Mr Putin said Russia did not rule out supporting a UN Security Council resolution authorising force, if it was proved “beyond doubt” that the Syrian government had used chemical weapons.
He said it was “too early” to talk about what Russia would do if America took action without a UN resolution.
Meanwhile, Jordan’s Prime Minister Abdullah Ensour told the BBC that it would back intervention in neighbouring Syria if proof emerged that chemical weapons had been used.
But he said any strikes would have to be precise, and that Jordan itself would not be involved.
‘World’s red line’
The government of Syrian President Bashar al-Assad is accused of using chemical weapons against civilians on several occasions during the 30-month conflict, most recently on a large scale in an attack on 21 August on the outskirts of Damascus.
The US has put the death toll from that incident at 1,429 – though other countries and organisations have given lower figures – and says all the evidence implicates government forces.
President Assad has said such an attack would have been “illogical” because UN chemical weapons experts were visiting Damascus at the time.
On Tuesday evening, senior members of the US Senate Committee on Foreign Relations agreed a draft resolution on military action to go before Congress – it specified that any operation would be “limited and tailored” and prohibit the use of any ground forces.
The full committee may vote on the draft on Wednesday.
Mr Obama has won the support of key Congressional leaders, though influential Republican John McCain said there were “number of people who are unhappy”.
In Stockholm, Mr Obama was asked if he believed asking Congress to vote – which he was not constitutionally obliged to do – had put his credibility at stake.
“My credibility is not on the line. The international community’s credibility is on the line,” he replied.
“America and Congress’s credibility is on the line, because we give lip-service to the notion that these international norms are important.”
Mr Obama, who has previously said that the use of chemical weapons would cross a “red line”, told reporters that it was not him who set this line but the world, “when governments representing 98% of the world’s population said the use of chemical weapons are abhorrent and passed a treaty forbidding their use even when countries are engaged in war”.
“Congress set a red line when it ratified that treaty,” he added.
He said he believed Congress would give its backing, because it recognised that the world would become “less safe” if chemical weapons were allowed to become the norm.
But he also stressed that as commander-in-chief, he had the right to act in his country’s national interest.
The United Nations says more than 100,000 people have been killed since the uprising against President Assad began in March 2011.
On Tuesday, the UN refugee agency said more than 2 million Syrians had now registered as refugees, and an estimated 4.25 million had been displaced within Syria, making it the worst refugee crisis since the 1994 genocide in Rwanda.
The foreign ministers of Lebanon, Jordan, Turkey and Iraq – which border Syrian and have taken in hundreds of thousands of refugees – are meeting at the UN in Geneva on Wednesday to discuss the humanitarian issue.
The ministers hope to persuade other richer countries to offer more support.