(CNN) Survivors of a deadly airstrike in Syria have described chemical bombs being dropped from planes, in accounts that directly contradicted the Assad regime's version of a dawn attack that drew condemnation around the world.

The strike, one of the deadliest of its kind since the Syrian war began six years ago, killed at least 70 people, including children.

Syria denied it used chemical weapons. Russia asserted the deaths resulted from a gas released when a regime airstrike hit a "terrorist" chemical weapons factory on the ground. But survivors being treated in a hospital on the Turkish side of the border told a CNN team they saw chemical bombs dropped from the air.

The World Health Organization said victims bore the signs of exposure to nerve agents, and Amnesty International said evidence pointed to an "air-launched chemical attack." International agencies are investigating the origin of the agents used in the strike.

Key developments

Chemical weapons expert: Russia's explanation of events is "highly implausible."

US President Donald Trump said the attack changed his views on Syria and Assad.

Medical experts said the attack was likely the result of a nerve agent, such as sarin gas.

The UN Security Council held an emergency meeting but didn't vote on a resolution.

War of words

A child receives treatment Tuesday at an Idlib province hospital after a suspected chemical attack.

Trump called the attack on innocent civilians an "affront to humanity," saying it had changed his views on Syria and Assad. "These heinous actions by the Assad regime cannot be tolerated," Trump said.

"It crossed a lot of lines for me. When you kill innocent children, innocent babies ... with a chemical gas that is so lethal that people were shocked to hear what gas it was, that crosses many, many lines -- beyond a red line."

The Russian Defense Ministry said on its Facebook page that a Syrian airstrike hit "workshops, which produced chemical warfare munitions" on the eastern outskirts of Khan Sheikhoun. It said "terrorists" had been transporting the chemical munitions from their largest arsenal to Iraq.

"Nerve agents are the result of a very expensive, exotic, industrial chemical process -- these are not something you just whip up," said Kaszeta, managing director of Strongpoint Security, a security consulting firm based in London.

"It's much more plausible that Assad, who's used nerve agents in the past, is using them again."

Hours after the attack, several people were injured when an airstrike hit near a hospital in the same town. Survivors of the earlier attack were being treated there, the Aleppo Media Center activist group reported.

The hospital was knocked out of service, said the Syrian Civil Defense rescue group, known as the White Helmets.

Victim: 'I don't know if my family is dead or alive'

Mazin Yusif, a 13-year-old-boy, broke down in tears at the Reyhanli Hospital in southern Turkey near the Syrian border. About 25 survivors of Tuesday's attack are being treated there, and several said they saw a plane drop chemical bombs.

Mazin Yusif, 13, tells a harrowing story of being caught up in the apparent chemical attack.

"At 6:30 in the morning, the plane struck. I ran up on our roof and saw that the strike was in front of my grandfather's house," Mazin told CNN.

He said he ran toward his house and found his grandfather slumped over. He ran outside to call for help. "I got dizzy and then fainted in front of my grandfather's garage. I next found myself here in this hospital, naked in a bed."

Aisha al-Tilawi, 55, says she lost three members of her family in Tuesday's attack.

The boy's grandmother, Aisha al-Tilawi, 55, said she saw blue and yellow after the plane dropped a chemical-laden bomb.

"We started choking, felt dizzy, then fainted. Mazin was trying to wake up his grandfather. Three of my family died," she said, lying in bed with an oxygen mask on her face.

Ahmed Abdel Rahim, 31, says he has no idea of his family's whereabouts after the attack.

Another survivor, Ahmed Abdel Rahim, 31, stared vacantly from his hospital bed while explaining he was hit with a poisonous substance carried by three rockets.

"I was in my house. I had difficulty breathing, but I feel better now. But I did throw up after getting to the hospital. I don't know if my family is dead or alive. I don't know anything," he said.

Worldwide condemnation

WHO said some victims showed symptoms consistent with exposure to a category of chemicals that includes nerve agents. That conclusion was supported by Amnesty International, which said victims were "very likely" to have been exposed to a compound such as sarin.

Photos: Suspected chemical attack in Syria Hide Caption 1 of 11 Photos: Suspected chemical attack in Syria This photo, provided Tuesday, April 4, by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun, Syria. Dozens of people were killed, according to multiple activist groups. Hide Caption 2 of 11 Photos: Suspected chemical attack in Syria A man carries a child at a makeshift hospital in Khan Sheikhoun. Activists have blamed the Syrian regime for the attack. In an apparent reference to the attack, two state-run Syrian media outlets reported that there was an explosion at a "rebel poison gas factory" in the countryside of Idlib province. The government has denied using chemical weapons in the past. Hide Caption 3 of 11 Photos: Suspected chemical attack in Syria A child's body is seen in Khan Sheikhoun. Hide Caption 4 of 11 Photos: Suspected chemical attack in Syria In this photo provided by the Idlib Media Center, doctors treat a child after the suspected attack. Hide Caption 5 of 11 Photos: Suspected chemical attack in Syria A baby's dead body is covered. Many of the casualties came as a result of asphyxiation, local doctors say. Hide Caption 6 of 11 Photos: Suspected chemical attack in Syria A dead baby is seen in the aftermath of the attack. "I've never seen anything like it, beyond description," said Fares al-Jundi, a doctor who spoke with CNN. He lives in a nearby village and rushed to the hospital after the airstrikes. Hide Caption 7 of 11 Photos: Suspected chemical attack in Syria Al-Jundi described how whole families were killed: They died of asphyxiation, and foam covered their mouths. Many died suddenly. "I believe this horrible memory will stay with me for the rest of my life," al-Jundi said. Hide Caption 8 of 11 Photos: Suspected chemical attack in Syria An unconscious Syrian child receives treatment at a hospital in Khan Sheikhoun. Hide Caption 9 of 11 Photos: Suspected chemical attack in Syria This photo provided by the Idlib Media Center shows victims of the suspected attack. Hide Caption 10 of 11 Photos: Suspected chemical attack in Syria Civil defense workers try to reduce the effects of toxic gas as they carry out search-and-rescue missions in Khan Sheikhoun. Hide Caption 11 of 11

The Organization for the Prohibition of Chemical Weapons said it was gathering evidence about the attack.

Speaking at a high-level meeting in Belgium on the future of Syria, UK Foreign Secretary Boris Johnson blamed the Syrian regime.

"All the evidence I have seen suggests that it was the Assad regime who did it, in full knowledge they were using illegal weapons in a barbaric attack on their own people," Johnson said.

Assad's military has repeatedly denied using chemical weapons and consistently blames "terrorist" groups when chemical attacks are reported.

But many of these are delivered through airstrikes, and no rebel or terrorist group in Syria is believed to have the capacity to carry out aerial bombardments.

A UN investigation in August found that chemical weapons had been used in Syria, both by the national air force and ISIS militants. It found two instances where regime forces had used chlorine as a chemical weapon, and one where ISIS had used mustard gas between 2014 and 2015.

The Syrian Coalition, an umbrella opposition group, compared this week's chemical attack to one in 2013 in eastern Ghouta "that the international community allowed to pass without accountability or punishment."

A UN report found that sarin had been used on civilians in the earlier attack that activists said killed about 1,400 people.

UN emergency meeting takes no action

At an emergency meeting of the UN Security Council, Nikki Haley, the US ambassador, held up photographs of children foaming at the mouth as evidence of the attack.

Syrian doctors treat a child following the suspected chemical attack Tuesday.

"If Russia has the influence in Syria that it claims to have, we need to see them use it," said Haley, who is the council president. "We need to see them put an end to these horrific acts. How many more children have to die before Russia cares?"

Haley hinted that the United States was open to using military action in response. "When the UN 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," she said.

Russia, Syria's most powerful ally, made clear it would not support a resolution put forward by the United States, the UK and France condemning the use of chemical weapons within Syria and requiring the regime to provide flight logs from the day of the attack.

Russia has used its veto power as a permanent member of the Security Council at least seven times on Syrian resolutions.

On Wednesday afternoon, Security Council members left closed consultations without taking a vote on the draft resolution. No member spoke as they left the chambers.

Syria's deputy ambassador to the United Nations, Mounzer Mounzer, flatly denied accusations his government is responsible, blaming "terrorist groups" for the dozens of deaths.

"Syria also reaffirms that the Syrian Arab Army does not have any form or type of chemical weapons," he said. "We have never used them, and we will never use them."