U.S. President Barack Obama has said the alleged use of chemical weapons in Syria in an attack on Wednesday, August 21 is a "big event of grave concern", BBC News reported.

Obama said the U.S. 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.