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President Barack Obama has said that the use of chemical weapons would cross a “red line” and carry “enormous consequences.” But he has tempered expectations that the U.S. would act swiftly since Washington first accused the Assad regime of carrying out chemical attacks, saying more information was needed before action could be taken.

In March, Britain and France told the U.N. chief they have reliable evidence that the Syrian government used chemical weapons near Aleppo, in Homs and possibly in Damascus.

The British and French ambassadors told Ban Ki-moon in a letter on March 25 that soil samples and interviews with witnesses and opposition figures backed their belief that the government used chemical shells that had caused injuries and deaths, diplomats and officials said. They spoke on condition of anonymity because the letter has not been made public.

Associated Press writer Jamal Halaby in Amman, Jordan, contributed reporting.