A top UN official was to visit Syria today for talks as demands grew for a prompt probe into opposition claims the regime unleashed a chemical attack that killed hundreds.
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 today, 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.
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