US Secretary of State John Kerry has signalled his country's intention to act on Syria, describing the use of chemical weapons as a "moral obscenity" and pinning the blame on the Syrian government.
In a strongly worded and emotive statement on Monday, Kerry said that it was "undeniable" that chemical weapons killed hundreds of people last Wednesday near Damascus, adding that the Syrian government must be held accountable.
The statement came hours after a United Nations team visiting Syria was fired upon while they travelled to the attack site to begin investigations.
"Let me be clear. 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," Kerry said.
"The meaning of this attack goes beyond the conflict in Syria itself. And that conflict has already brought so much terrible suffering. This is about the large-scale indiscriminate use of weapons that the civilised world long ago decided must never be used at all, a conviction shared even by countries that agree on little else," Kerry added.
In apparent reference to the Syrian government's denials of responsibility for the attack, Kerry said "anyone who could claim that an attack of this staggering scale could be contrived or fabricated needs to check their conscience and their own moral compass".
"Our sense of basic humanity is offended not only by this cowardly crime but also by the cynical attempt to cover it up. At every turn, the Syrian regime has failed to cooperate with the UN investigation, using it only to stall and to stymie the important effort to bring to light what happened in Damascus in the dead of night," Kerry added.
He said that the belated response to allow UN inspections was "too late, and too late to be credible".
According to US law, President Barack Obama has the authority to launch air strikes against Syria, but he has to notify Congress first - a process which has begun, according to both sides.
"The administration is actively consulting with members of Congress, and we will continue to have these conversations in the days ahead," Kerry said in his statement on Syria.
The Republican speaker of the House Of Representatives, John Boehner, has had "preliminary communication with the White House about the situation in Syria and any potential US response," his spokesman, Brendan Buck, said.
"The speaker made clear that before any action is taken there must be meaningful consultation with members of Congress, as well as clearly defined objectives and a broader strategy to achieve stability," Buck said.
Al Jazeera's Patty Culhane, reporting from Washington, said Obama had not made any final decisions but it seems clear from Kerry's comments that the US is putting itself in a position where it will have to do something.
"There's been a lot of talk about some sort of targetted strikes with cruise missiles that could be handled by the four US Navy destroyers that are off the coast of Syria," our correspondent said.
"What the White House will say is the president is not even considering putting in what they call 'boots on the ground' - meaning the idea of putting US troops into Syria isn't even on the table."
Russia's Foreign Minister Sergei Lavrov on Monday said military intervention in Syria without a UN mandate would be a grave violation of international law.
"We've had this movement in Iraq and Libya and not a single case of military interference and intervention resulted in things improving or bringing stabilisation. The region is destabilised in an unprecedented way so everyone should act with utmost responsibility and work together, as was agreed by leaders during the G8 summit in June this year," Lavrov said.
The US State Department on Monday announced it had postponed a meeting scheduled for Wednesday in The Hague between senior diplomats from the US and Russia due to "ongoing consultations" over the alleged chemical weapons attack.
"We will work with our Russian counterparts to reschedule the meeting," a State Department official said, adding that the attack demonstrated the need for a "comprehensive and durable political solution" to end the bloodshed.
Warplanes in Cyprus
Kerry did not say what action the US administration would take, but said that Obama would be making an "informed decision about how to respond".
"He believes there must be accountability," Kerry said.
In-depth coverage of escalating violence across Syria
Meanwhile, warplanes and military transporters have been seen arriving at Britain's Akrotiri airbase on Cyprus, less than 160km from Syria's coastline, according to a report on the website of Britain's Guardian newspaper.
Two commercial pilots who fly from the city of Larnaca, near the base, told the Guardian they had seen C-130 transport planes from their cockpit windows and small formations of fighter jets on their radar screens, which they told the newspaper they believed had flown from Europe.
Residents also reported to the newspaper that there had been more activity around the base than usual. If Western powers were to launch attacks on Syria, Cyprus would likely be central to any air campaign.