This photo provided Tuesday, April 4, 2017 by the Syrian anti-government activist group Edlib Media Center, which has been authenticated based on its contents and other AP reporting, shows Syrian doctors treating a child following a suspected chemical attack, at a makeshift hospital, in the town of Khan Sheikhoun, northern Idlib province, Syria. The suspected chemical attack killed dozens of people on Tuesday, Syrian opposition activists said, describing the attack as among the worst in the country's six-year civil war. (Edlib Media Center, via AP)

This photo provided Tuesday, April 4, 2017 by the Syrian anti-government activist group Edlib Media Center, which has been authenticated based on its contents and other AP reporting, shows Syrian doctors treating a child following a suspected chemical attack, at a makeshift hospital, in the town of Khan Sheikhoun, northern Idlib province, Syria. The suspected chemical attack killed dozens of people on Tuesday, Syrian opposition activists said, describing the attack as among the worst in the country's six-year civil war. (Edlib Media Center, via AP)

BEIRUT (AP) — A chemical weapons attack in an opposition-held town in northern Syria killed dozens of people on Tuesday, leaving residents gasping for breath and convulsing in the streets and overcrowded hospitals. The Trump administration blamed the Syrian government for the attack, one of the deadliest in years, and said Syria’s patrons, Russia and Iran, bore “great moral responsibility” for the deaths.

The Britain-based Syrian Observatory for Human Rights said at least 58 people died, including 11 children, in the early morning attack in the town of Khan Sheikhoun, which witnesses said was carried out by Sukhoi jets operated by the Russian and Syrian governments.

Videos from the scene showed volunteer medics using fire hoses to wash the chemicals from victims’ bodies. Haunting images of lifeless children piled in heaps reflected the magnitude of the attack, which was reminiscent of a 2013 chemical assault that left hundreds dead and was the worst in the country’s ruinous six-year civil war.

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Tuesday’s attack drew swift condemnation from world leaders, including President Donald Trump, who denounced it as a “heinous” act that “cannot be ignored by the civilized world.” The U.N. Security Council scheduled an emergency meeting for Wednesday in response to the strike, which came on the eve of a major international donors’ conference in Brussels on the future of Syria and the region.

In a statement, Trump also blamed former President Barack Obama for “weakness” in failing to respond aggressively after the 2013 attack.

“These heinous actions by the Bashar al-Assad regime are a consequence of the past administration’s weakness and irresolution,” Trump said. “President Obama said in 2012 that 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 attack.”

Trump left it to his top diplomat, Secretary of State Rex Tillerson, to assign at least some blame to Russia and Iran, Assad’s most powerful allies. Tillerson called on both countries to use their influence over Assad to prevent future chemical weapons attacks, and noted Russia’s and Iran’s roles in helping broker a cease-fire through diplomatic talks in the Kazakh capital, Astana.

“As the self-proclaimed guarantors to the cease-fire negotiated in Astana, Russia and Iran also bear great moral responsibility for these deaths,” Tillerson said.

In a statement, the Syrian government “categorically rejected” claims that it was responsible, asserting that it does not possess chemical weapons, hasn’t used them in the past and will not use them in the future. It laid the blame squarely on the rebels, accusing them of fabricating the attack and trying to frame the Syrian government.

The Russian Defense Ministry said the Khan Sheikhoun residents were exposed to toxic agents from a rebel arsenal hit by a Syrian air strike.

The ministry spokesman, Maj. Gen. Igor Konashenkov, said in a statement early Wednesday that Russian military assets registered the strike on a weapons depots and ammunition factory on the town’s eastern outskirts. Konashenkov said the factory produced chemical weapons that were used in Iraq.

Photos and video emerging from Khan Sheikhoun, located south of the provincial capital of Idlib, showed the limp bodies of children and adults. Some were struggling to breathe; others appeared to be foaming at the mouth.

The activist-run Assi Press published video of paramedics carrying victims, stripped down to their underwear and many appearing unresponsive, from the scene in pickup trucks.

It was not immediately clear if all those killed died from suffocation or were struck by other airstrikes that occurred in the area around the same time.

It was the third claim of a chemical attack in just over a week in Syria. The previous two were reported in Hama province, in an area not far from Khan Sheikhoun.

Opposition activists and a doctor in Idlib said it was the worst incident since the 2013 gas attack on the Damascus suburb of Ghouta that killed hundreds of civilians and which a U.N. investigation said used sarin gas.

Faced with international outrage over that attack, Assad agreed to a Russia-sponsored deal to destroy his chemical arsenal. His government declared a 1,300-ton stockpile of chemical weapons and so-called precursor chemicals that can be used to make weapons, all of which were destroyed.

But member states of the Organization for the Prohibition of Chemical Weapons have repeatedly questioned whether Assad declared everything. The widely available chemical chlorine was not covered in the 2013 declaration and activists say they have documented dozens of cases of chlorine gas attacks since then.

The Syrian government has consistently denied using chemical weapons and chlorine gas, accusing the rebels of deploying it in the war instead.

Dr. AbdulHai Tennari, a pulmonologist who treated dozens of victims of Tuesday’s attack, said it appeared to be more serious than a chlorine attack.

In a Skype interview, he said doctors were struggling amid extreme shortages, including of the antidote used to save patients, Pralidoxem.

Most of the fatalities died before they reached hospitals, Tennari said. “If they got to the hospital we can treat them. Two children who took a while before they were lifted out of the rubble died,” he said.

Dr. Mohammed Tennari, a radiologist and AbdulHaj Tennari’s brother, said Tuesday’s attack was more severe than previous ones in the province, most of which used chlorine cylinders.

“Honestly, we have not seen this before. The previous times the wounds were less severe,” he said. The doctor, who testified before the United Nations in 2015 about renewed Syrian government use of chemical attacks despite claims it has destroyed its stockpiles, said there was a chlorine smell after Tuesday’s attack, but it was mixed with another unknown “toxic gas which causes poison and death.”

Mohammed Hassoun, a media activist in the nearby town of Sarmin, where some of the critical cases were transferred, said doctors there also believed it was likely more than one gas. “Chlorine gas doesn’t cause such convulsions,” he said, adding that doctors suspect sarin was used.

“There are 18 critical cases here. They were unconscious, they had seizures and when oxygen was administered, they bled from the nose and mouth,” he told The Associated Press.

Tarik Jasarevic, spokesman for the World Health Organization in Geneva, said by email that the agency was gathering more information about Tuesday’s incident. The Syrian American Medical Society, which supports hospitals in opposition-held territory, also said it had sent inspectors to Khan Sheikhoun and an investigation was underway.

Hussein Kayal, a photographer for the Idlib Media Center, said he was awakened by the sound of a bomb blast around 6:30 a.m., and when he arrived at the scene he found entire families inside their homes unable to move, with their eyes wide open and their pupils constricted. He put on a mask, and he and others took victims to an emergency room. He said he later felt a burning sensation in his fingers and was treated for that.

The province of Idlib, which is almost entirely controlled by the opposition, is home to some 900,000 displaced Syrians, according to the United Nations. Rebels and opposition officials have expressed concerns that the government is planning to mount a concentrated attack on the crowded province.

New York-based Human Rights Watch has accused the Syrian government of conducting at least eight chlorine gas attacks on opposition-controlled residential areas during the final months of the battle for Aleppo last year that killed at least nine civilians and injured 200. A joint investigation by the United Nations and the international chemical weapons watchdog determined the Syrian government was behind at least three additional attacks in 2014 and 2015 involving chlorine gas, and that the Islamic State group was responsible for at least one, involving mustard gas.

Late Tuesday, Abu Hamdu, a rescue worker in Khan Sheikhoun, said people were still searching for their family members, nearly 12 hours after the attack.

“People are still very lost,” he said.

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Associated Press writers Philip Issa in Beirut, Vladimir Isachenkov in Moscow, Vivian Salama and Josh Lederman in Washington, and Mike Corder in The Hague, Netherlands, contributed to this report.