International leaders have condemned the alleged chemical attacks which killed at least 58 people, including 11 children in Syria's Idlib province on Tuesday.
Syrian regime jets pounded the town of Khan Shaykhun. Medics attending the victims said they bore the signs of a gas attack.
The war monitor Syrian Observatory for Human Rights (SOHR) said those killed had died from suffocation and the effects of a toxic gas.
The weapons used caused many people to choke or faint, and some had foam coming out of their mouths, SOHR said, citing medical sources who described the air strikes as a "toxic gas attack."
Fifteen victims, mostly women and children, were brought to Turkey for treatment.
Turkey's Foreign Minister Mevlut Cavusoglu said Turkey condemns the suspected chemical attack, describing it as a crime against humanity.
He said the attack could derail the process of Syrian peace talks being held in Astana, Kazakhstan.
EU's diplomatic chief Federica Mogherini said the regime of Bashar al Assad bears "primary responsibility" for the attack.
"Today the news is awful," Mogherini said in an interview with media organisations in Brussels on the sidelines of a EU-UN conference that was meant to focus on the post-conflict situation in Syria.
French President Francois Hollande accused Assad regime of responsibility for a "massacre".
"Once again the Syrian regime will deny the evidence of its responsibility for this massacre," Hollande said in a statement.
The White House also blamed the attack on the regime, saying the incident was "reprehensible and cannot be ignored by the civilised world."
"These heinous actions by the Bashar al-Assad regime are a consequence of the last administration's weakness and irresolution," White House spokesman Sean Spicer told a briefing.
British Foreign Secretary Boris Johnson also condemned the attack saying those responsible should be "held to account."
Drawing criticism from around the world, #Idlib was also trending globally on social media site, Twitter.
Russia denies involvement
Turkish President Recep Tayyip Erdogan and Russian President Vladimir Putin discussed Tuesday's attack, Turkish presidential sources said.
"President Erdogan touched on the use of chemical weapons against civilians in Idlib. President Erdogan said such inhumane attacks are unacceptable," a statement from top residential sources said.
However, Russia has categorically denied any involvement.
"Planes of the Russian air force have not carried out any strikes near Khan Shaykhun of Idlib province," said a statement by the Russia's defence ministry.
The two leaders also have emphasised the importance of the maintenance of an ongoing ceasefire in Syria.
TRT World's Shamim Chowdhury has the latest from Gaziantep near the Turkish-Syria border.
Opposition demands UN investigation
Syria's opposition National Coalition demanded a UN investigation into the attack.
"The National Coalition demands the [UN] Security Council convene an emergency session ... open an immediate investigation and take the necessary measures to ensure the officials, perpetrators and supporters are held accountable," the body said in a statement.
The opposition said on Tuesday that the attack "calls into question" the peace process.
"If the United Nations cannot deter the regime from carrying out such crimes, how can it achieve a process that leads to political transition in Syria?" said Mohamad Sabra, the head negotiator for the main opposition High Negotiations Committee (HNC) that participated in recent peace talks in Geneva.
The regime military source denied that its forces used any such weapons, saying the army "does not and has not" used chemical weapons "not in the past and not in the future."
Meanwhile, a rocket has slammed into a hospital in northwestern Syria on Tuesday as doctors treated victims of the suspected chemical attack, destroying part of the building.
The projectile hit the building in Khan Shaykhun, bringing down rubble on top of medics.
TRT World spoke to Idlib-based Syrian activist Ahmad Ibrahim.