UN Secretary-General Ban Ki-moon has sent Under Secretary General Angela Kane to Damascus for talks, his spokesman Eduardo del Buey said in a statement.
"It is his intention to conduct a thorough, impartial and prompt investigation on the reports of the alleged use of chemical weapons during these attacks."
The statement said the UN chief had already written to Damascus to request its cooperation in investigating Wednesday's reported attack.
"The Secretary-General urges the Syrian authorities to respond positively and promptly to his request without delay," it said, adding that Ban had also called on Syrian rebels to cooperate with the mission.
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If confirmed, it would mark the deadliest use of chemical weapons since Saddam Hussein deployed poison gas against Iranian troops and Kurdish rebel areas in the 1980s.
Footage distributed by activists showed unconscious children, people foaming around the mouth and doctors apparently administering oxygen to help them breathe.
The team of UN inspectors arrived in Syria this week to investigate three sites where chemical weapons were previously alleged to have been used.
The Syrian government has denied using chemical weapons, and its close ally Russia has accused rebels of fabricating the attack.