United Nations war crimes investigators have opened a probe of a suspected chemical attack in Syria's Idlib Province that left at least 58 people dead.
The UN Commission of Inquiry for Syria said on April 4 that it had opened the probe in a statement condemning the attack.
The statement also condemned the alleged targeting of a medical facility where victims of the attack were being treated, saying both actions "would amount to war crimes and serious violations of human rights law."
UN Syrian envoy Staffan de Mistura told journalists in Brussels that all indications were that "it was a chemical attack and it came from the air."
The United Nations Security Council will convene an emergency session on April 5 to discuss the incident.
International outrage at the apparent attack continued to mount throughout the day.
U.S. President Donald Trump issued a statement saying the attack was "reprehensible and cannot be ignored." He blamed the "heinous" actions of Syrian President Bashar al-Assad on the "weakness and irresolution" of his predecessor, Barack Obama.
U.S. Secretary of State Rex Tillerson said that Russia and Iran -- as Assad's main allies and guarantors of the Astana cease-fire process -- "bear great moral responsibility" for the Idlib attack. He called on Moscow and Tehran to use their influence and ensure that no more similar attacks occur.
French President Francois Hollande also said Assad bore responsibility for the "massacre" and called on "those who support this regime…[to] once again reflect on the enormity of their political, strategic, and moral responsibility."
British Foreign Secretary Boris Johnson said that if it was proven the Syrian government carried out a chemical attack, it would be "unquestionably a war crime and they must be held to account."
WATCH: EU foreign policy chief Federica Mogherini says those behind a suspected chemical attack in a rebel-held town in Syria must be held accountable.
Turkish President Recep Tayyip Erdogan condemned the attack as an "inhuman" strike that could endanger peace talks. Turkish presidential sources said Erdogan made the comments in a phone call with his Russian counterpart, Vladimir Putin.
The Syrian government has denied that it carried out a chemical attack, saying that it destroyed a "terrorists' chemical-weapons depot" in the town of Khan Shekhoun. The government statement said the attack was carried out using Russian-made Sukhoi Su-22 fighter bombers, which are not capable of deploying chemical weapons.
The Syrian military also denied using chemical weapons.
The London-based Syrian Observatory for Human Rights and the Syrian Union of Medical Care Organizations said the death toll in the attack could be as high as 100, with up to 400 injured. Many of the injured are reportedly suffering respiratory problems.
Earlier, the observatory said symptoms among those affected by the attack included fainting, vomiting, and foam coming out of their mouths. It was unknown what chemical was used in the attack, but local medics were quoted as saying they suspected it was the nerve agent sarin.
The reported chemical attack came ahead of an April 5 international conference in Brussels on the future of Syria and the region.
At least 300,000 people have been killed and millions displaced by Syria's 6-year-old civil war. A UN-led investigation has concluded that the Syrian government used chlorine as a weapon at least three times in 2014 and 2015.