"President [Barack] 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 in the most forceful US reaction yet to the 21 August attack (see background).
Speaking after UN chemical weapons experts came under sniper fire on their way to investigate the scene of the attack, White House spokesman Jay Carney said the use of chemical weapons was undeniable and "there is very little doubt in our mind that the Syrian regime is culpable."
Kerry said Obama was consulting with allies before deciding how to respond.
"What we saw in Syria last week should shock the conscience of the world," Kerry told reporters.
"The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it is inexcusable. And despite the excuses and equivocations that some have manufactured, it is undeniable."
Military chiefs from the US and its European and Middle Eastern allies met in Jordan for what could be a council of war, should they decide to punish Assad, who has denied using chemical weapons and blamed rebels for staging such attacks.
UN investigators crossed the front line from the center of the capital, which remains under Assad's control, to inspect the Mouadamiya suburb, one of at least four neighborhoods hit by the poison gas before dawn last Wednesday.
The UN said one vehicle in its convoy was crippled by gunshots fired by "unidentified snipers." The team continued on after turning back for a replacement car.
Syrian state television blamed rebel "terrorists" for the shooting. The opposition blamed pro-Assad militiamen.
Kerry said Assad's decision to finally allow access to inspectors was too late to be credible. "That is not the behavior of a government that has nothing to hide," Kerry said, adding that Assad's forces had also destroyed evidence by shelling the area.
"Our sense of basic humanity is offended not only by this cowardly crime, but also by the cynical attempt to cover it up," Kerry said.
At most, the UN inspectors could confirm that chemical weapons were used, not who used them, he added, but it was Assad's government that has such weapons and the means of delivering them.
Military speculation mounts
Speculation has mounted that Western countries will order some kind of military response to an incident that took place a year after Obama declared the use of chemical weapons a "red line" that would require strong action.
With his international credibility seen increasingly on the line, Obama could opt for limited measures such as cruise missile strikes to punish Assad and seek to deter further chemical attacks, without dragging Washington deeper into the war. The United States has started a naval buildup in the region to be ready for Obama's decision.
In neighboring Israel, citizens have been queuing up for gas masks in case Assad responds to a Western attack by firing on Israel, as Iraq's Saddam did in 1991.
With tensions rising over Syria, British Prime Minister David Cameron cut short a holiday to lead a top-level security meeting. In the past few days, Obama, Cameron and French President François Hollande have all spoken to each other in a flurry of phone calls. Cameron also called Russian President Vladimir Putin on Monday (26 August).
Several NATO countries have issued statements pledging a response, although none has been specific about what is planned.
Assad denies the accusations that his forces used chemical weapons and said the United States would be defeated if it intervened in his country.
"Would any state use chemicals or any other weapons of mass destruction in a place where its own forces are concentrated? That would go against elementary logic," he told the Russian newspaper Izvestia. "Failure awaits the United States as in all previous wars it has unleashed, starting with Vietnam and up to the present day."
Russia, Assad's main arms supplier and diplomatic defender in the UN Security Council, says rebels may have been behind the chemical attack. Foreign Minister Sergei Lavrov said any intervention in Syria without a Security Council resolution would be a grave violation of international law.
French Foreign Minister Laurent Fabius noted that Russia and China would probably veto a UN Security Council vote to allow strikes against Syria. But British Foreign Secretary William Hague said it would still be possible to respond to a chemical weapons attack without the Security Council's permission.
There are precedents. In 1999 NATO attacked Serbia, a Russian ally, without a Security Council resolution, arguing that action was needed to protect civilians in Kosovo.
Turkey, a NATO ally and major backer of Syria's opposition, said it would join any international coalition even if a decision for action could not be reached at the United Nations.