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 inquiry.
A spokesman for UN Secretary General Ban Ki-Moon said he was "giving his utmost attention to the tragic situation" and intended to conduct a "thorough, impartial and prompt investigation".
The UN High Representative for Disarmament Affairs, Angela Kane, will travel to Damascus on Saturday to push for access for the UN inspectors.
"It is of paramount importance that all those who share the concern and urgency of investigating these allegations, equally share the responsibility of co-operating in generating a safe environment for the [UN] mission to do its job," Mr Ban's spokesman added.
Damascus has described the allegations that it sanctioned the use of chemical weapons as "illogical and fabricated".
The main opposition alliance, the National Coalition, has meanwhile said that it will do everything to assist the UN inspectors and ensure their safety.
"It is critical that those inspectors get there within 48 hours. The clock is ticking and we want to see those inspectors and we believe that the evidence will show who used those chemical weapons against innocent civilians," spokesman Khaled Saleh said.
Opposition activists are also reportedly trying to smuggle tissue samples from victims' bodies to the UN inspectors to prove their claims.
"The UN team spoke with us and since then we prepared samples of hair, skin and blood and smuggled them back into Damascus with trusted couriers," activist Abu Nidal told the Reuters news agency.
UK Foreign Secretary William Hague said it seemed like the Syrian government had "something to hide" over the reported chemical attack.
"I know that some people in the world would like to say this is some kind of conspiracy brought about by the opposition in Syria. I think the chances of that are vanishingly small and so we do believe that this is a chemical attack by the Assad regime on a large scale," he told reporters.
French Foreign Minister Laurent Fabius has said that there should be a "reaction of force" from the international community if allegations that the Syrian government was responsible are verified.
'Chemical attack': What we know
- 01:15: 21 August (10:15 GMT 20 Aug): Facebook pages of Syrian opposition report heavy fighting in rebel-held districts in the eastern Ghouta, the agricultural belt around Damascus
- 02:45: Opposition posts Facebook report of "chemical shelling" in Ein Tarma area of Ghouta
- 02:47: Second opposition report says chemical weapons used in Zamalka area of Ghouta
- Unverified video footage shows people being treated on pavements in the dark and in makeshift hospital
- Reports say chemical weapons were used in Ghouta towns of Irbin, Jobar, Zamalka, and Ein Tarma as well as Muadhamiya in the western Ghouta, but this is not confirmed
- Syrian government acknowledges military offensive in the Ghouta but denies chemical weapons use
Meanwhile, the UN says children now make up half of all refugees fleeing Syria. About three-quarters of those children are under 11.
"The youth of Syria are losing their homes, their family members and their futures," said UN high commissioner for refugees Antonio Guterres.
Most of the children have arrived in Lebanon, Jordan, Turkey, Iraq and Egypt, the UN says, and increasingly Syrians are fleeing to North Africa and Europe.
But with its appeal for Syria less than 40% funded, the two UN agencies say they are struggling to meet the needs of the refugees.
The BBC's Imogen Foulkes in Geneva says the agencies are now warning of a lost generation that are ill-equipped to bring peace and stability to their country in the future.