Obama: Syria chemical weapon claim a 'grave concern' 23 August 2013 Last updated at 17:05 Bridget Kendall investigates how the attack unfolded, mainly through extremely distressing videos posted online 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. "The notion that the US can somehow solve what is a sectarian, complex problem inside of Syria sometimes is overstated" - Video from CNN 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. Very troublesome 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. Barack Obama says the attack is "a big event of grave concern" that is "going to require America's attention" if proved true. But it is worth reading the whole interview with the president to get a sense of his extreme caution. It is uncertain what action the US might take - curiously the state department has said that previously the red lines have been crossed and the US has taken action, but that they can't say what. You might think a private punishment is not much of a deterrent and anything that happens now will have to be a lot more public. But Mr Obama does not sound like a man gung ho for military action. It sounds like the pleading of man being dragged, pushed and pulled by allies and world opinion to do something but who wants to be certain it doesn't end up in a new war. 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've seen people with loss of speech... [one child] compulsively shooting with a toy gun," said UN High Commissioner for Refugees Antonio Guterres 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 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. Damascus has described the allegations that it sanctioned the use of chemical weapons as "illogical and fabricated." But according to Reuters news agency, Syrian anti-government activists are 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. The main opposition alliance, the National Coalition, also said it would 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 Reuters. '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.