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. (AFP)