In harsh, uncompromising language, Secretary of State John Kerry began laying out the U.S. case for possible military action against Syria, saying there was undeniable evidence that chemical weapons had been used in a deadly attack against a rebel enclave and that it was "a moral obscenity."

Obama administration planning centers on carrying out any U.S. and allied strikes on Syria as part of a coalition without United Nations backing, U.S. and European officials said. Such a route could raise international law...