US President Barack Obama has said the alleged use of chemical weapons in Syria in an attack on Wednesday is a “big event of grave concern”.
Mr Obama said the US was still seeking confirmation such weapons were used, but if proved true the situation would “require America’s attention”.
Meanwhile, Syria’s main ally Russia has said there is growing evidence that Syrian rebels were behind the attack.
The opposition says hundreds died in a government assault outside Damascus.
But despite calls from many different countries, there is no sign yet that the Syrian authorities will allow a UN inspection team to visit to investigate the claims.
Unverified footage shows civilians – many of them children – dead or suffering from what appear to be horrific symptoms as a result of Wednesday’s attack.
Also on Friday, UN agencies said the number of children forced to flee Syria had reached one million.
The UN’s refugee agency, UNHCR, and children’s fund, Unicef, described the figure as “a shameful milestone”, and said a further two million children were displaced within the country.
Last year, President Obama said the use of chemical weapons in Syria would cross a “red line” and force a tough US response.
In an interview broadcast on CNN on Friday, he said that the recent claims of chemical weapons use were “very troublesome”.
“What we’ve seen indicates clearly this is a big event, of grave concern, and we are already in communications with the entire international community,” Mr Obama said.
He said that “core national interests” of the US were involved in the Syrian conflict, “both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region.”
The BBC has learnt that some in the White House are furious and regard Wednesday’s attack as an outrage that breached international law and demands a response.
But Mr Obama warned in his interview: “Sometimes what we’ve seen is that folks will call for immediate action, jumping into stuff, that does not turn out well, gets us mired in very difficult situations, can result in us being drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region.”
Meanwhile, Russia joined calls for an “objective investigation” by UN chemical weapons experts.
Foreign Ministry spokesman Aleksandr Lukashevich said Moscow had urged President Bashar al-Assad to co-operate with a probe, but also that questions remained about the willingness of the opposition to provide “secure, safe access of the [UN] mission to the location of the incident”.
“More new evidence is starting to emerge that this criminal act was clearly provocative,” the ministry added.
“On the internet, in particular, reports are circulating that news of the incident carrying accusations against government troops was published several hours before the so-called attack. So, this was a pre-planned action.”
The ministry also described as “unacceptable” calls from various European capitals for the UN Security Council to authorise the use of force in Syria.
‘Something to hide’
Other leaders have also pushed for an urgent UN investigation.
UN Secretary General Ban Ki-moon has already called for inspectors to start “without delay”.
“I can think of no good reason why any party – either government or opposition forces – would decline this opportunity to get to the truth of the matter,” Mr Ban said from the South Korean capital, Seoul.
He said any use of chemical weapons would violate international law and should result in “serious consequences for the perpetrator”.
UK Foreign Secretary William Hague is holding discussions with Mr Ban, US Secretary of State John Kerry and Qatar’s foreign minister to press for immediate access for the UN team.