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”.
Barack 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 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 Barack Obama said the use of chemical weapons in Syria would cross a “red line” and force a tough US response.
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 authorize the use of force in Syria.
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,” Ban Ki-Moon’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 told the Reuters news agency.
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 Reuters.