US President Barack Obama said the alleged use of chemical weapons was "a big event of grave concern," while Russia hit out at calls for force against its ally Syria.
US Defense Secretary Chuck Hagel strongly suggested the Pentagon was moving forces into place ahead of possible military action against Syria, even as Obama voiced caution.
US commanders have nevertheless prepared a range of "options" for Obama if he chooses to proceed with military strikes against Damascus, Hagel told reporters aboard his plane en route to Malaysia.
"The Defense Department has a responsibility to provide the president with options for all contingencies," Hagel said.
"And that requires positioning our forces, positioning our assets to be able to carry out different options -- whatever the president might choose."
But Hagel declined to provide any details on the positioning of US ships, aircraft or troops, as the Obama administration reportedly contemplated cruise missile strikes against Assad's forces.
Hagel's comments came as a defense official said the US Navy would expand its presence in the Mediterranean with a fourth warship armed with cruise missiles.
Britain accused Damascus of unleashing the weapons and France called for "force" if the claims were confirmed.
UN chief Ban Ki-moon stepped up the pressure by announcing that Under Secretary General Angela Kane was headed to Damascus for talks, his spokesman said.
Kane was due to arrive on Saturday, and Ban is determined to "conduct a thorough, impartial and prompt investigation" into the chemical claims, a statement said.
"The Secretary-General urges the Syrian authorities to respond positively and promptly to his request without delay," it said, adding that Ban also called on Syrian rebels to cooperate with the mission.
Opponents of President Bashar al-Assad said his forces used chemical weapons east and southwest of Damascus in attacks Wednesday that killed hundreds.
The regime has strongly denied the accusations.
Activists released harrowing footage showing unconscious children, people foaming around the mouth and doctors apparently giving them oxygen has triggered revulsion around the world.
British Foreign Secretary William Hague blamed Syria and demanded it grand immediate access to UN inspectors who have been in the country since Sunday to probe three other sites.
"We do believe this is a chemical attack by the Assad regime on a large scale, but we would like the United Nations to be able to assess that," he said.
Russia urged Damascus to cooperate with the UN but dismissed calls for use of force against its ally.
The foreign ministry said Internet footage distributed by the opposition said to implicate the regime had been posted "several hours before the so-called attack."
"Against the background of another anti-Syrian wave of propaganda, we believe calls from some European countries to apply pressure on the UN Security Council and already now take a decision on the use of force are unacceptable," it said.
The growing alarm was reflected in separate phone conversations that Russian Foreign Minister Sergei Lavrov had with his US, British and French counterparts.
Lavrov and US Secretary of State John Kerry agreed on the need for an "objective investigation", said the Russian foreign ministry.
Syria's main opposition National Coalition pledged to guarantee the safety of the UN inspectors, warning that the "clock is ticking" before alleged evidence vanishes.
"We will ensure the safety of the UN inspectors team, however, it is very critical to get that team into the area that was just hit in less than 48 hours, the clock is ticking," spokesman Khaled Saleh said.
So far, the Syrian government has not said whether it will let the inspectors visit the sites.
The National Coalition says more than 1,300 people were killed in gas attacks southwest and east of the capital.
Obama said the allegations were more serious than previous ones against Assad's regime.
"We are right now gathering information about this particular event," he said, while warning against the United States intervening hastily and getting "mired in very difficult situations".
One year ago, Obama warned the use of chemical weapons in Syria would cross a "red line" and have "enormous consequences".
Damascus denied any wrongdoing saying it would be "political suicide" to unleash a chemical attack when UN inspectors are in Syria.
The United Nations says more than 100,000 people have been killed in Syria since March 2011, while millions more have fled the country or been internally displaced.
As the regime's allies and foes traded barbs, UNICEF said one million children have fled Syria in what the UN children's agency called a "tragic milestone" in the 29-month conflict.
"One million is more than the number of children living in Wales. One million children is more than the number of children living in Los Angeles and Boston combined," said UN high commissioner for refugees Antonio Guterres.