Mr Obama said the US was still seeking confirmation² 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 opposition³ 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 inspection⁴ team to visit to investigate the claims.

 

Unverified footage shows civilians⁵ - 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 shameful⁶ milestone⁷", 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 proliferating⁸, 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 outrage⁹ that breached¹⁰ international law and demands a response.

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