President Obama said Friday the United States was consulting with allies and considering how to respond if an investigation determines that Syria used chemical weapons in an alleged attack on rebel-held villages in the eastern Damascus suburbs of Ghouta this week.

“Although the situation in Syria is very difficult…there is no doubt that when you start seeing chemical weapons used on a large scale… then that starts getting to some core national interests that the United States has, 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,” Obama told CNN’s Chris Cuomo in an interview aired Friday.

“We are right now gathering information about this particular event,” Obama continued, “but I can say that, unlike some of the evidence that we were trying to get earlier that led to a UN investigator going into Syria, what we've seen indicates that this is clearly a big event of grave concern.”

While the US, Europeans and Russia have called on the Syrian government to allow UN inspectors access to the site, “we don’t expect cooperation, given their past history,” Obama said.