The United Nations has formally asked the Syrian government to authorise UN experts to investigate a reported chemical attack with mass casualties near Damascus.

UN Secretary General Ban Ki-moon has also decided to send Angela Kane, the UN High Representative for Disarmament Affairs to Damascus, UN spokesman Eduardo del Buey said.

Damascus agreed at the end of July, following an earlier visit by Kane, to allow UN experts to investigate three other sites where chemical weapons attacks were alleged to have taken place.

“The secretary-general believes that the incidents reported yesterday need to be investigated without delay,” del Buey said in a statement.

“The secretary-general now calls for the mission, presently in Damascus, to be granted permission and access to swiftly investigate the incident which occurred on the morning of 21 August 2013.

“A formal request is being sent by the United Nations to the government of Syria in this regard. He expects to receive a positive response without delay,” del Buey said.

The UN chief also called for a cessation of hostilities so humanitarian aid could reach the victims of the fighting.

The UN spokesman gave no details on when Kane would travel.

UN officials said the head of the UN experts team in Damascus, Ake Sellstrom of Sweden, has already begun negotiations with the Syrian authorities.

The UN Security Council on Wednesday supported the UN demand for an investigation, stressing that a “thorough, impartial and prompt investigation” was needed to shed light on the incident.

The main Syrian opposition coalition has claimed that President Bashar al-Assad’s regime killed at least 1300 people in toxic gas attacks on rebel strongholds outside Damascus on Wednesday.