(FOX NEWS) A year after President Obama warned the Syrian regime that using chemical weapons would cross a “red line,” Syrian activists are accusing Bashar Assad’s forces of killing hundreds in a gas attack — in what would be the worst and deadliest chemical attack to date.

The attack raises new concerns that the so-called “red line” in Syria has been crossed, violated and effectively ignored. The situation compounds the Mideast challenges facing the Obama administration, as his national security team continues to deliberate on how to handle the unrest in Egypt — another trouble spot where the U.S. is keeping itself at a distance.

White House spokesman Josh Earnest said in a statement Wednesday that the U.S. is “deeply concerned” by the reports in Syria and is calling for a United Nations probe.

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