Video shows effects of Syria attack

(CNN)Editor's Note: This story contains graphic imagery.

Dozens of people, including at least ten children, have been killed in what is suspected to be one of the deadliest chemical attacks in Syria in years, multiple activist groups say.

Airstrikes hit the rebel-held city of Khan Sheikhoun in Idlib province on Tuesday morning, giving off a "poisonous gas," according to Anas al-Diab, an activist with the Aleppo Media Center (AMC).

A man carries a child into a makeshift hospital after the attack.

The attack, which has been blamed on the Syrian regime by activists and condemned by international leaders, has reportedly left hundreds injured.

Many of the casualties came as a result of asphyxiation, local doctors say. Videos circulating on social media purporting to be from the scene show people, including children, who appear unresponsive; others are seen struggling to breathe or wearing oxygen masks.

A child receives treatment following the suspected gas attack.

A doctor in a hospital close to Khan Sheikhoun told CNN: "Today around 7:30 a.m., about 125 injuries arrived to our hospital. Twenty-five of them were already dead, 70 to 80% of the wounded people were kids and women.

"The symptoms were pale skin, sweating, narrow or pin-eye pupils, very intense respiratory detachments. Those symptoms match the usage of sarin."

The doctor, who cannot be named for security reasons, said tests were needed to identify for certain if sarin gas was to blame.

Another doctor, Fares al-Jundi, lives in a nearby village and rushed to the hospital after the airstrikes.

He told CNN: "I've never seen anything like it, beyond description."

Al-Jundi estimated there were around 500 wounded people. They covered the floors of the entire hospital, from the patients' rooms to the operating rooms and the corridors.

The doctor, who was choked with emotion as he spoke, described how whole families were killed, mothers and their children. They died of asphyxiation, foam covered their mouths. Many died suddenly, he said.

"I believe this horrible memory will stay with me for the rest of my life."

Victims of the suspected chemical attack.

The AMC estimated that the death toll from the attack was 70. The UK-based Syrian Observatory for Human Rights said at least 58 were dead, including ten children. The High Negotiations Committee, an umbrella opposition group, claimed the death toll could be as high as 100.

Several people were injured later Tuesday when an airstrike hit near a hospital in Khan Sheikhoun, where victims from the earlier apparent gas attack were being treated, the AMC reported.

The nature of the substance has not been officially confirmed, and it is unclear if the planes involved were Syrian.

A Syrian man receives treatment.

Idlib province is largely controlled by an alliance of Syrian rebel forces and is regularly targeted in airstrikes by the Syrian government and its ally, Russia.

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.

The Russian Defense Ministry said none of their jets carried out airstrikes in the area of Khan Sheikhoun Tuesday, according to a ministry statement carried by state-run TASS.

A child is treated following the attack on Tuesday.

The Syrian Coalition, an umbrella opposition group, referred to the suspected chemical attack as a "crime similar to that in Eastern Ghouta in 2013 that the international community allowed to pass without accountability or punishment."

The tweet was in reference to a 2013 chemical attack on the Damascus suburb of Ghouta, which was blamed on the Syrian regime and prompted then-US President Barack Obama to threaten military action.

If Tuesday's reports of a chemical attack by the regime are confirmed, experts say it could be the biggest since Ghouta.

McCain: Assad must pay a price

US Senator John McCain called Tuesday for President Donald Trump to send a strong signal to Syrian President Bashar al-Assad that the attack will not be tolerated.

McCain referred to the 2013 chemical attack Tuesday, telling CNN: "We've seen this movie before, it was when Barack Obama said they would have a red line, they crossed it and he did nothing. Bashar al-Assad and his friends, the Russians, take note of what Americans say. "

The Ghouta attack prompted Obama to ask Congress to authorize military action against Syria in 2013.

Shortly after, Syria agreed to a Russian proposal to give up control of its chemical weapons, leading Obama to quietly retreat from threats of military action against the regime.

McCain called for Trump — who is yet to comment on what happened Tuesday — to say "we're going to arm the Free Syrian Army, we're going to dedicate ourselves to the removal of Bashar Assad, we're going to have the Russians pay a price for their engagement. "

International condemnation

The news from Idlib came as an international conference opened Tuesday in Brussels to discuss the future of Syria.

It is being hosted by the European Union and co-chaired with the UN and with the governments of Germany, Kuwait, Norway, Qatar and the United Kingdom.

Speaking at a press conference with his German counterpart, UK Foreign Secretary Boris Johnson suggested the attack could amount to a war crime.

"If this were to be proved to have been committed by the Assad regime then it would be another reason to think they are an absolutely heinous outfit, that is, it is a war crime," Johnson said.

Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan had a phone conversation Tuesday morning "where Erdogan said chemical weapons used against civilians in Idlib and those cruel attacks are not acceptable," Turkey's official news agency Anadolu reported.

The Turkish Foreign Minister Mevlut Cavusoglu tweeted: "Idlib attack is a crime against humanity. Those saying Syrian people will decide Assad's future: no people will remain if attacks continue."

French Foreign Minister Jean-Marc Ayrault called for an emergency meeting of the UN Security Council, describing the attack as a "despicable act" and "testimony to the barbarism which the Syrian people have been victims of for many years" in a statement.

The Organization for the Prohibition of Chemical Weapons (OPCW) said it was "seriously concerned" about the reports from Idlib, and said a fact-finding mission had been launched into the incident.

Israeli Prime Minister Benjamin Netanyahu said he was "shocked and outraged" in a series of tweets. "There's no, none, no excuse whatsoever for the deliberate attacks on civilians and on children, especially with cruel and outlawed chemical weapons," he wrote in one.

Bana Alabed, the 7-year-old Syrian girl who became famous for her tweets from the besieged city of Aleppo last year, tweeted at President Trump.

"This is today in Syria, in #Idlib. Hi @realdonaldtrump do you love this?" she wrote, with a photo showing a pile of children stripped down to their underwear — some with their eyes closed.

Analyst: Regime may increase attacks

A man is aided by civil defense workers in Idlib on Tuesday.

Haid Haid, a Syrian columnist and researcher, told CNN that in his view, the US is "not interested in any way challenging the rule of Assad."

"The regime has been systematically using small scale [chemical] attacks over a long time in order to normalize the use of those attacks," claimed Haid, an associate fellow at Chatham House, who works on the Middle East and North Africa Programme.

"Once the regime is sure that the attacks won't trigger any sort of reaction from the international community, I believe the regime will start increasing those attacks."

He added: "The attack we are discussing right now is bigger in scale than many other attacks in the past few years, and it's so far the biggest since the one in 2013, which means that the regime is in a way increasing the scale of the attacks in order to test the waters to see if the international community will do anything to address what's happening inside Syria."

Since the war began in 2011, an estimated 400,000 Syrians have been killed, according to figures issued by the United Nations last year.

CNN's Milena Veselinovic, Tamara Qiblawi, Richard Roth, Kristina Sgueglia, Hande Atay Alam, Judith Vonberg and Nick Thompson also contributed to this report.

McCain on Syria: We've seen this movie before