President tells press briefing 'we have evidence' of use of weapons but says 'what we don't know is who used them'

Barack Obama has made his clearest threat yet of international action against Syria, if the US can confidently establish that Bashar al-Assad's government was responsible for the recent alleged use of chemical weapons in the country.

However, speaking at the White House after days of ambiguous rhetoric from Washington, the president said that he did not yet believe there was sufficient evidence to trace the use of chemical weapons back to President Assad's government.

"What we now have is evidence that chemical weapons have been used inside Syria," Obama told reporters. "What we don't know is who used them. We don't have a chain of custody. Without evidence of what happened, how can I make a decision what to do? I have got to make sure I have got the facts."

The president made clear that it felt this was important not just to avoid repeating the mistakes made by the US over claims of the presence of weapons of mass destruction in Iraq, but also because it would need to convince more countries to join it in any action against Syria.

"If we rush to judgment without hard evidence we will find ourselves in a position where we cannot mobilise the international community for what we have to do," said Obama. "It is important that we do this in a prudent way."

However, he also admitted that Pentagon and other military planners had since last year been working on a range of options for retaliation, which would be implemented if Washington could establish "a clear base line of facts". Officials in Damascus have insisted publicly that the Syrian government has not been responsible for the use of chemical weapons.

"If I can establish the use of chemical weapons by the Assad regime in a way that the US and international community can be sure of, that is a game changer," Obama said. "By 'game changer' I mean we would have to review the range of options that are available. There are options at the moment that are on the shelf, but it is a spectrum of options and [use of chemical weapons] clearly would be an escalation of the threat."

Secretary of State John Kerry recently warned members of Congress in private that US military options against Syria are limited. Imposing a no-fly zone would do little to contain the chemical-weapons threat and would also risk exposing US pilots to "highly effective" air defenses. Targeting facilities with special forces or high-temperature incendiary bombs would be difficult, because such facilities are thought to be dispersed.

Politicians briefed by Kerry said they felt the most likely option would be to join allies such as the UK and France in selectively arming certain rebel groups within Syria.

Obama, who has been under pressure from Congress for being slow to act over the Syrian civil war, stressed in Tuesday's press briefing that he had not been standing idly by.

"For several years what we have been seeing is a slowly unfolding disaster for the Syrian people and we have not simply been bystanders," he said. "We are the largest humanitarian donor. We have been providing non-lethal assistance to the rebels. Even if chemical weapons were not being used in Syria we would still be talking about a regime that has killed tens of thousands of its own people."

However, he hinted that any use of chemical weapons by the Syrian government would be seen as unacceptable – not just deployment on a large scale.

"Use of chemical weapons would be a game changer," said Obama. "We don't want the genie out of the bottle."