A massive chemical weapons attack hit an area of the northern Idlib province in Syria on Tuesday, followed by a bombing run on hospitals where victims were undergoing treatment.

According to Fox News and the Associated Press, Syrian medical relief organization UOSSM reported least 100 people were killed and 400 more injured. Some other estimates put the death toll as low as 57. The London-based Syrian Observatory for Human Rights added that at least 11 children were among the dead.

The Organization for the Prohibition of Chemical Weapons is reportedly investigating the incident.

The UK Independent reports that an AFP journalist saw rubble crashing down on the medical staff treating patients at one hospital, while as-yet-unverified footage showed an explosion striking a hospital while a reporter was interviewing doctors on-camera.

“The Syrian Civil Defence rescue group said two of its centres were targeted by air strikes – one in Khan Sheikhoun and one in nearby Habit. The group said none of its volunteers were injured but the damage put the facilities out of service,” the Independent writes. Khan Sheikhoun is about 30 miles south of the city of Idlib.

The BBC interviewed a reporter and ambulance driver who observed people choking in the streets after the chemical weapon strikes. The relief service this ambulance driver works for published photographs depicting “what appeared to be at least seven dead children in the back of a pick-up truck,” with no sign of traumatic injuries on their bodies.

One AFP journalist reported seeing “a young girl, a woman and two elderly people dead at a hospital, all with foam still visible around their mouths.” The facility where this gruesome tableau was observed was subsequently hit by a rocket strike, burying doctors under rubble.

A Syrian medical activist reported at least 18 “critical cases” of unconscious people bleeding from the nose and mouth to the Associated Press. Another eyewitness, a photographer for the Idlib Media Center, said he found “entire families inside their homes, lying on the floor, eyes wide open and unable to move.” He reported feeling a burning sensation after he touched their clothing.

The BBC notes these symptoms are consistent with the use of sarin gas, which causes death by asphyxiation within a matter of minutes, and is very difficult to detect. The Syrian government has allegedly used sarin gas on previous occasions. The regime of dictator Bashar Assad denied using the gas and blamed rebel fighters for deploying it.

CBS News reports the Syrian and Russian governments “flatly denied any responsibility” for the attack. The Russian defense Ministry denied conducting any airstrikes on the Khan Sheikhoun area, while Syrian officials “repeated blanket denials of ever having used chemical weapons,” and in fact denied targeting anyone except “terrorists” at all.

CBS posted Tweets from a man identifying himself as a British-trained physician performing volunteer work in northern Syria, claiming he found evidence of a Sarin gas attack:

Do u still doubt that #Sarin is being used on us? Non-reactive pinpoint pupils! We have samples. Will anyone care!? Who will stop it?#Syria pic.twitter.com/WmhDZgLVA6 — Dr Shajul Islam (@DrShajulIslam) April 4, 2017

Several British analysts and officials also told CBS they believed a nerve agent was deployed in the attack, while the British government formally stated that photos from Khan Sheikhoun “strongly suggest the use of chemical weapons.” One analyst said the symptoms resembled the notorious Ghouta attack of 2013 that crossed President Barack Obama’s rhetorical “red line” against WMD. The upshot of that epic Obama administration debacle was Syria supposedly destroying its chemical weapons stockpile under the watchful eye of Russia.

British Foreign Secretary Boris Johnson declared himself “horrified” and said the attack “bears all the hallmarks” of the Syrian government. He stated the British government will “continue to lead international efforts to hold perpetrators to account.”

Britain and France have called for an emergency meeting of the U.N. Security Council to discuss the attack, and urged Russia and China not to block international action against the responsible parties.

Israeli President Benjamin Netanyahu quickly weighed in on the attack. He said the “shocking pictures” from Idlib should “shake every human being.”

“Israel strongly condemns the use of chemical weapons in any situation, especially against innocent civilians. Israel calls on the international community to fulfill promises made in 2013, and to remove chemical weapons from Syria,” Netanyahu said.

The Jerusalem Post quotes former Israeli military intelligence chief Amos Yadlin calling upon Israel to “establish a more active policy against the negative developments” and make Assad “pay for his war crimes.” He complained of the void in global authority left by Obama’s “red line” collapse, and said the void was filled by “Russia, the Islamic State, and Hezbollah.”

“Today’s chemical attack in Syria against innocent people including women and children is reprehensible and cannot be ignored by the civilized world. These heinous actions by the Bashar al-Assad regime are a consequence of the past administration’s weakness and irresolution,” said White House Press Secretary Sean Spicer on Tuesday.

“President Obama said in 2012 he would establish a red line against the use of chemical weapons and then did nothing. The United States stands with our allies across the globe to condemn this intolerable act,” Spicer continued.