He said what was witnessed in Syria last week should "shock the conscience of the world".
"It defies any code of morality...the indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity."
"The meaning of this attack goes beyond the conflict on Syria itself."
The US secretary of state accused president Bashar Assad of destroying evidence.
Ratcheting up criticism of Syria's alleged chemical weapons use, Mr Kerry called last week's attack a "moral obscenity".
He said the US has additional information about the attack and will make it public in the days ahead.
Mr Kerry said shelling the affected area afterwards was not the action of a government trying to co-operate with UN investigators trying to assess what happened.
The alleged chemical weapons attack on August 21 that killed hundreds of people has moved the US closer to military action against Syria than at any point during the bloody civil war.
Assad has denied launching a chemical attack.
It comes as international tensions worsened when UN weapons inspectors came under sniper fire as they sought to investigate a deadly attack blamed by Western nations on the Assad regime.
Prime Minister David Cameron is breaking off from his holiday to continue a round of calls with world leaders amid mounting speculation that the UK could join international military action.
He will chair a meeting of the National Security Council on Wednesday to discuss potential responses to the deadly attack which killed hundreds but Foreign Secretary William Hague declined to rule out air strikes within days.
Force may be the only remaining option after the failure of diplomatic efforts to end the bloody violence in Syria, he said, insisting that it could be deployed legally even without Russian support at the UN Security Council.
The inspectors were travelling to the site of the attack in a suburb of Damascus after the regime - which blames any chemical weapon use on opposition forces - gave its permission almost a week after the incident.