A chemical attack that killed scores of civilians in Syria probably involved a banned nerve agent, top medical groups concluded Wednesday, as the United States and European allies at the U.N. Security Council demanded an investigation.

The attack on the northwestern town of Khan Sheikhoun on Tuesday was the deadliest chemical assault on a civilian population in Syria since government forces­ attacked rebel-held suburbs of Damascus with sarin nerve agent in 2013.

In southern Turkey, witnesses evacuated for treatment described being enveloped by a fog of chemicals and said that rescue workers passed out among the people they were trying to help.

“Some people died as they were sleeping. The ones who did not, especially the children, were foaming at the mouths and shaking,” said a resident, Hussein al-Nimr. “We tried to rescue people, but then we fell down, too.”

Back in Khan Sheikhoun, families spoke of grief and a sense of betrayal.

“If the world wanted to stop this, they would have done so by now,” a woman who gave her name as Om Ahmed said in a telephone interview. “One more chemical attack in a town the world hasn’t heard of won’t change anything.”

Then her voice cracked. “I’m sorry. My son died yesterday,” she said. “I have nothing left to say to the world.”

At least 72 people were killed in the attack, according to the Britain-based Syrian Observatory for Human Rights monitoring group. Medical organizations working inside Syria said they had treated more than 500 people who inhaled the still unidentified chemical.

The World Health Organization said victims showed symptoms consistent with a reaction to a nerve agent, stockpiles of which President Bashar al-Assad’s government was supposed to have destroyed more than three years ago in an attempt to stave off U.S. military intervention.

Addressing the Security Council meeting in New York on Wednesday, U.S. Ambassador Nikki Haley hinted that the Trump administration may now be considering escalation once again.

[At U.N., Trump administration assails Russia for protecting Assad]

“When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action,” Haley said. Hours later at the White House, President Trump said that “heinous actions by the Assad regime cannot be tolerated.”

1 of 14 Full Screen Autoplay Close Skip Ad × The scene in Syria after a chemical attack kills dozens View Photos At least 72 reported dead after a chemical attack on a northern Syrian town. Caption The White House said it had evidence of preparations for another chemical attack in Syria and warned the Syrian regime against it. Earlier this year, dozens died after a chemical attack in a northern town. April 4, 2017 A man is helped by Syrian Civil Defense workers following a toxic gas attack in Khan Sheikhoun, a rebel-held town in the northwestern Idlib province. Mohamed Al-Bakour/AFP via Getty Images Buy Photo Wait 1 second to continue.

Turkey’s health minister, Recep Akdag, said Wednesday that 30 Syrians have been evacuated across the border for treatment in Turkish hospitals. Outside a facility in the southern city of Reyhanli, a small crowd of Syrians waited all day for news, their cellphones glued to their ears as they relayed updates to relatives back in Khan Sheikhoun.

Hussam Abu Ammash, 43, clutched a list of 21 names of relatives he said were killed in the attack. “When people woke up and saw the cloud, they thought it was the dust of an airstrike and they ran toward it. But it was chemical gases,” he said.

Doctors Without Borders said its medics treated patients with dilated pupils, muscle spasms and involuntary defecation “consistent with exposure to neuro­toxic agents such as sarin.”

Long-standing global treaties have banned the use of chemical weapons, including sarin, a substance that can kill within minutes.

Doctors and activists across opposition-held areas have cited a sharp increase in chemical attacks since government forces­ routed rebels from their longtime stronghold of Aleppo in mid-December. The Syrian Network for Human Rights monitoring group said Wednesday that it has recorded nine separate strikes since the start of the year.

Many of those involved chlorine, which is less deadly than a nerve agent.

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While Tuesday’s assault was widely attributed to the Syrian government, Russia tried Wednesday to shift the blame to armed groups opposing Assad.

Maj. Gen. Igor Konashenkov, a Russian military spokesman, said Syrian warplanes had been targeting rebel workshops on the eastern outskirts of Khan Sheikhoun, which is in rebel-held Idlib province. “The territory of this storage facility housed workshops to produce projectiles filled with toxic agents,” he said in a recorded statement.

His comments marked a rare admission that airstrikes had taken place in the area. Moscow typically denies knowledge of such mass-casualty attacks and has previously falsified video footage in an attempt to exonerate its military.

Russia also blamed the 2013 sarin attack on rebels who it said were attempting to provoke international intervention.

But Syrian rebel commander Hasan Haj Ali told the Reuters news agency that the Russian assertion was “a lie.” He said that the rebels do not have the capability to produce chemical weapons and that no military positions in the area were bombed. “Everyone saw the plane while it was bombing with gas,” he said.

The Hague-based Organization for the Prohibition of Chemical Weapons (OPCW), which is charged with ensuring that nations live up to the weapons ban, would lead any investigation on the ground in Syria in partnership with the United Nations. At the time of the 2013 sarin attack, an OPCW team was already in Syria and was able to travel to the site within five days, collecting soil and blood samples. Several weeks later, it issued a formal report alleging the use of sarin nerve agent, although it never formally blamed Assad.

This time, rescue workers said they had gathered soil samples from Khan Sheikhoun and sent them to Western intelligence officials for analysis.

The Syrian government has denied any involvement in Tuesday’s attack or use of chemical weapons.

The attack came amid an upswing in Syrian government strikes in Idlib, to which hundreds of thousands of civilians have fled from other battle zones around the country. With the Turkish border to the north largely closed, they are now trapped.

Zakaria reported from Reyhanli. Andrew Roth in Moscow, Michael Birnbaum in Brussels and Brian Murphy in Washington contributed to this report.

Read more:

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Even after so many deaths, chemical attack in Syria provoked visceral response

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