CNN  — 

About 500 people may have been affected by a suspected chemical attack last week on Syria’s rebel-held town of Douma, according to the World Health Organization.

It remains difficult to place an exact figure on the number of people killed and wounded Saturday on the outskirts of Damascus, a development that has pushed the United States and its allies to the verge of military confrontation in Syria.

But in a statement Wednesday, WHO, citing reports from its health partners, said an estimated 500 patients seen at medical facilities exhibited symptoms consistent with exposure to toxic chemicals.

These symptoms included respiratory failure, severe irritation of mucous membranes and disruption to the central nervous system.

WHO said more than 70 people taking shelter in basements reportedly died in the attack, with 43 of those deaths related to exposure to toxic chemicals.

Two health facilities were also affected, its statement said.

WHO demanded “immediate unhindered access to the area” and said it was on standby to provide assistance to those affected, as soon as access was granted.

The statement comes one day after the United States and Russia, Syria’s key ally, blocked each other’s proposals for an independent investigation during a heated UN Security Council meeting.

Russian Lt. Gen. Viktor Poznikhir said Wednesday that military police would be sent to Douma on Thursday. “Russian military police units will be deployed to the city of Douma to ensure security, maintain law and order and arrange assistance to the local population,” Poznikhir told reporters.

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The Organisation for the Prohibition of Chemical Weapons has said it will convene a meeting Monday in The Hague, Netherlands, to discuss the alleged use of chemical weapons in Syria. It is also set to carry out a fact-finding mission in Douma, though it has so far declined to provide a timetable.

According to activist groups, helicopters dropped barrel bombs filled with toxic gas on the last rebel-held town in Eastern Ghouta, which has been besieged for six years and the focus of a renewed government offensive launched in mid-February.

Graphic footage shot by rescuers and activists show victims – including children – dead and injured, some ghostly white and foaming at the mouth in makeshift clinics. Others were found suffocated in their homes, according to first responders.

CNN has not been able to verify the authenticity of the images independently.

Children use respirators following a suspected chemical attack in Douma on Saturday night.

The attack has drawn condemnation from Western leaders, with President Donald Trump vowing Monday to come to a decision “over the next 24 to 48 hours” on how to make Syria pay a “big price.” And on Wednesday he warned Russia it should “get ready” for a missile strike on Syria, vowing to thwart any missile defenses.

But a decision on how to respond to the chemical attack in Syria had not yet been made when Trump issued his tweet, people familiar with knowledge of the discussions said.

The Daily Telegraph in London reported Wednesday that UK Prime Minister Theresa May has ordered British submarines to move within missile range of Syria, in preparation for strikes against the Assad regime that could begin as early as Thursday night. A British government official told CNN that, though preparations are underway, the political decision has not yet been made.

Whitehall sources told The Daily Telegraph that Britain was “doing everything necessary” to be able to fire Tomahawk cruise missiles from nuclear-powered submarines against military targets in Syria. One source told the Telegraph “if any action is going to happen, it is going to happen before Monday.” The UK’s Ministry of Defence refused to comment on the report to CNN.

May will host a Cabinet meeting on Thursday “to discuss the response to Syria,” a Downing Street spokesperson confirmed to CNN.

The Syrian government and Russia vehemently deny involvement and accuse rebel groups of fabricating the attack to hinder the army’s advances and provoke international military intervention. On Wednesday, Poznikhir alleged that the White Helmets – a Syrian civil defense group – had “staged chemical attacks for the cameras” and that Russian chemical weapons specialists and medical personnel had found no trace of chemical weapons in the affected area.

Eastern Ghouta, one of the last major rebel-held areas in Syria, was once home to around 400,000 people. But in the past four weeks alone more than 133,000 people have fled, with tens of thousands more trapped inside its largest town, Douma, UN refugee agency spokesman Andrej Mahecic said in a statement Tuesday.

CNN’s Milena Veselinovic and Nathan Hodge contributed to this report.