By Kareem Khadder, Schams Elwazer, Elizabeth Roberts, Eyad Kourdi and Jomana Karadsheh CNN

(CNN) -- 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).

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

Doctor: 'I've never seen anything like it'

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 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."

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.

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.

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 referring 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

Turkey's official news agency Anadolu reported that 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."

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

The news from Idlib Tuesday came as an international conference opened 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.

UK Foreign Secretary Boris Johnson reacted to the news by writing on Twitter: "Horrific reports of chemical weapons attack in Idlib Syria. Incident must be investigated & perpetrators held to account."

Meanwhile French Foreign Minister Jean-Marc Ayrault called for an emergency meeting of the UN Security Council.

"A new, particularly serious chemical attack targeted the province of Idlib this morning. Initial reports indicate a very large number of victims, including children. I strongly condemn this despicable act," Ayrault said in a statement.

He added that the use of chemical weapons constitutes an unacceptable violation of the Convention on the Prohibition of Chemical Weapons (CWC), saying that the attack was "a new testimony to the barbarism, which the Syrian people have been victims of for many years."

Analyst: Regime may increase attacks

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."

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