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A suspected chemical attack killed dozens of people in a rebel-held area of Syria on Tuesday, activists said, describing an incident that would mark one of the worst of its kind in Syria's six-year civil war.

At least 83 people died — including 25 children — and at least 350 others were injured in the incident in northern Idlib province, southwest of Aleppo, the relief agency UOSSM told NBC News. UOSSM, formally known as the Union of Medical Care and Relief Organizations, is a nonprofit coalition of groups treating victims of the war in Syria.

President Donald Trump said the attack was "reprehensible and cannot be ignored by the civilized world." He blamed it on the regime of Syrian President Bashar al-Assad and the Obama administration's "weakness and irresolution" to act after initially citing the use of chemical weapons as a "red line."

The Syrian government denied any involvement and said it was complying with the 1997 Chemical Weapons Convention, which bans such instruments of war, according to SANA, the Syrian state-run news agency. Instead, the government blamed "armed terrorist organizations" for the attack.

"The Syrian Arab republic stresses that all those fabricated allegations will not prevent it from continuing its war on terrorism ... and from working for a political solution to the crisis in Syria," SANA reported.

The Organization for the Prohibition of Chemical Weapons, which monitors the chemical weapons treaty, said it had set up a fact-finding mission to get to the bottom of Tuesday's attack.

The pro-opposition Idlib Media Center posted pictures to Facebook showing children who appeared to be either dead or incapacitated, and the White Helmets, a volunteer group, shared images of its members spraying children with a hose.

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NBC News has not verified the photos, but government leaders, including U.S. Secretary of State Rex Tillerson, said the attack clearly used banned chemical weapons.

Tillerson noted that the attack was the third time such weapons are alleged to have been used this month, charging: "It is clear that this is how Bashar al-Assad operates: with brutal, unabashed barbarism.

"Those who defend and support him, including Russia and Iran, should have no illusions about Assad or his intentions," Tillerson said in a statement. "Anyone who uses chemical weapons to attack his own people shows a fundamental disregard for human decency and must be held accountable."

Related: Syrian Government Blamed for Third Chemical Attack: Report

British Foreign Secretary Boris Johnson said in a statement: "Although we cannot yet be certain about what has happened, this bears all the hallmarks of an attack by the regime which has repeatedly used chemical weapons." British Prime Minister Theresa May described the attack as "barbarism."

Israeli Prime Minister Binyamin Netanyahu tweeted that he was "shocked and outraged."

When I saw pictures of babies suffocating from a chemical attack in Syria, I was shocked and outraged > — Benjamin Netanyahu (@netanyahu) April 4, 2017

U.N. Security Council President Nikki Haley, the U.S. ambassador to the United Nations, said the Security Council would meet to discuss the attack on Wednesday morning.

A senior Syrian military officer, meanwhile, told NBC News on condition of anonymity that the government was not responsible.

"Syrian forces and its allies target terrorist groups, not civilians," the officer said, adding that his country would "continue the fight against terror until victory is accomplished."

The government has consistently denied using chemical weapons and chlorine gas, accusing the rebels of deploying it, instead. Assad agreed to destroy Syria's chemical weapons in the wake of an international outcry after a sarin nerve gas attack on the Damascus suburb of Ghouta killed hundreds of civilians in 2013.

2013: Syria's Chemical Weapons Arsenal Remains a Menacing Mystery

A senior State Department official said, meanwhile, that if the attack is determined to have been carried out by Assad's government, Russia and Iran will also "obviously have a lot to answer for."

"Russia has said it had nothing to do with the airstrikes in Syria today, but that’s not the issue," the official said in a conference call with reporters. "The issue is an apparent inability or unwillingness to hold the regime to its own commitments and to account."

2015: Chemical Weapons Used by Fighters in Syria: OPCW Report

Rep. Tulsi Gabbard, D-Hawaii, who was heavily criticized by members of both parties for flying to Syria and meeting with Assad in January, called the attack a "war crime" on Twitter.

In January, Gabbard urged the United States to end what she called its "regime change" effort in Syria. But she said Tuesday that whoever was responsible for the attack "must be held accountable," even if it turns out to have been Assad's government.

(1/3) Horrific Syria chemical attack is a war crime that must be thoroughly condemned. — Tulsi Gabbard (@TulsiGabbard) April 4, 2017

(2/3) Whoever is found responsible, be it the Syrian govt, al-Qaeda, or ISIS, (all have access to chemical weapons) must be held accountable — Tulsi Gabbard (@TulsiGabbard) April 4, 2017