Ratcheting up criticism of Syria's alleged chemical weapons use, Kerry called last week's attack a "moral obscenity" that should shock the conscience of the world.
He says the U.S. has additional information about the attack and will make it public in the days ahead.
Kerry says shelling the affected area afterward was not the action of a government trying to cooperate with U.N. investigators trying to assess what happened.
The alleged chemical weapons attack on Aug. 21 that killed hundreds has moved the U.S. closer to military action against Syria than at any point during the bloody civil war.
Syrian President Bashar Assad has denied launching a chemical attack.