"Our sense of basic humanity is offended not only by this cowardly crime but also by the cynical attempt to cover it up," a tight-jawed Kerry said in a statement Monday in Washington.
"By any standard, it is inexcusable and -- despite the excuses and equivocations that some have manufactured -- it is undeniable," Kerry said of last week's "indiscriminate slaughter of civilians" in Damascus suburbs in which between a reported 355 to 1,100 civilians died and another 3,600 people were injured.
President Obama has made it clear to the Assad regime "that this international norm cannot be violated without consequence," the top U.S. diplomat said.
He said he and Obama were discussing the situation with their international counterparts and members of Congress to come up with a thoughtful, informed response.
Kerry said additional "real and compelling" evidence about the attack would be made available in the days ahead.
"President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people," Kerry said.
He said the five days between the attack and when the Assad regime decided to allow U.N. chemical weapons inspectors access to the site showed that the Syrian government was trying to hide what happened.
"If the regime had nothing to hide, their response should be immediate transparency ... not shelling," an angry Kerry said. "Failure to commit to that would tell its own story."
"What we saw in Syria last week should shock the conscience of the world," Kerry said. "It defies any code of morality. ... The indiscriminate slaughter of civilians ... by chemical weapons is a moral obscenity."
The "gut-wrenching" images circulating in traditional and social media all point to one thing, Kerry said: "That chemical weapons were used in Syria."
Assad has denied that troops loyal to him used chemical weapons and threatened dire consequences should his country be subjected to an airstrike.
"The area of the claimed attack is in contiguity with the Syrian Army positions, so how is it possible that any country would use chemical weapons in an area where its own forces are located?" he asked in an interview with Russian newspaper Izvestia.
Russia, one of a handful of allies to Syria, has accused rebels of staging the attack.
"Anyone who can claim that an attack of this staggering scale ... was staged ... needs to check their conscience," Kerry said.
CNN reported there was an explosion near the site the inspectors planned to visit. Some witnesses said it was caused by incoming ordnance.
The team of U.N. inspectors probing the possible use of chemical weapons in Syria spent its first day in Damascus suburbs at the site of the alleged incident, interviewing witnesses, survivors and doctors, and collecting samples, Secretary-General Ban Ki-moon said Monday.
The team returned to the site after its convoy was hit by sniper fire earlier this morning. Though its vehicle was destroyed, the team, led by Swedish scientist Ake Sellstrom, was unharmed. Ban said the U.N. will register a "strong complaint" with the Syrian government and opposition authorities about an attack "so the safety ... of the investigation team will be secured."
"What I am told is that despite the very difficult circumstances, our team replaced their car and returned to the suburbs of Damascus to carry out their investigation," he said.
U.S. Defense Secretary Chuck Hagel said U.S. forces were "prepared to exercise whatever option" Obama ordered.
British Foreign Secretary William Hague would not discount bombing regime targets inside Syria soon, warning a diplomatic effort to resolve the crisis hasn't borne fruit.
William Hague also insisted Britain, the United States and France were united to act against Assad, The Daily Telegraph reported Monday.
Iran predicted "harsh consequences" if the United States intervened against the Iranian ally.
Israeli President Shimon Peres called for an international effort to "take out" Syrian chemical weapons.