It also demanded "unhindered access" to Douma in Syria. The Syrian government denies any use of chemical weapons, the Guardian reported.
"WHO demands immediate unhindered access to the area to provide care to those affected, to assess the health impacts and to deliver a comprehensive public health response," Peter Salama, the agency's Deputy Director General for emergency preparedness and response, said in a statement issued in Geneva.
The US has threatened a "forceful" response to reports of an attack but Russia has called this a "pretext" to attack its ally.
The WHO cited reports that 70 people were killed in the attack. It said its "health cluster" partners reported that 500 people had suffered symptoms consistent with exposure to toxic chemicals, including difficulty breathing, irritation of mucous membranes, and disruption to the central nervous system.
"Two health facilities were also reportedly affected by these attacks," the WHO statement added.
Syrian opposition activists, rescue workers and medics alleged that the attack on Douma was carried out by government forces using bombs filled with toxic chemicals.
The attack appeared to have prompted a surrender deal the following day by local rebels. Jaysh al-Islam, the dominant opposition group in Douma, agreed to leave the town along with thousands of civilians for northern Syria, after weeks of saying they had no intention of agreeing to a deal, the daily said.
Since then, thousands of the estimated 100,000 people still living in Douma have left on buses heading north. More were expected to leave on Wednesday.
Douma was the last holdout in eastern Ghouta after other rebel groups agreed to leave other parts of the region, which has been under siege for years and subjected to multiple chemical attacks in the past.
UN High Commissioner for Human Rights Zeid Ra'ad al-Hussein said the world must react to the use of chemical weapons or risk dire consequences.
"After decades when we thought we had successfully outlawed the use of chemical and biological weapons, the world is sitting idly by while their use is becoming normalised in Syria," said Al-Hussein.
The French representative at the UN Security Council said poison gas had deliberately been used as it could seep down to the basements, the BBC reported.
Following the alleged attack, Syria and Russia reached an evacuation deal with the Jaish al-Islam rebels, who up until now have been holding Douma.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
