The United States government believes the chemical agent sarin was used in Tuesday's chemical attack in Syria's Idlib province, a government source told Reuters on Tuesday.
The source added that it was "almost certainly" carried out by forces loyal to Syrian President Bashar Al-Assad.
At least 100 people were killed and 400 more were injured in Tuesday's attack.
A human rights organization in Syria quoted medical sources who treated victims of the attack as saying that the victims were suffering from fainting spells and suffocation. Others were suffering from palpitations and other symptoms of gas attacks.
The Syrian military denied responsibility and said it would never use chemical weapons.
The Organization for the Prohibition of Chemical Weapons (OPCW) has in the past found that some people in Syria may have been exposed to sarin or a sarin-like gas.
U.S. Secretary of State Rex Tillerson, meanwhile, reacted to the chemical weapons attack and said it shows how Assad operates with "brutal and 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 quoted by Reuters.
"Anyone who uses chemical weapons to attack his own people shows a fundamental disregard for human decency and must be held accountable," he added.
The Secretary of State Rex Tillerson said that Russia and Iran bear "great moral responsibility" for the deaths from the chemical weapons attack, because they declared themselves to be the guarantors of a ceasefire they helped broker in Kazakhstan.
The White House said earlier on Tuesday that the Obama administration is responsible for creating conditions allowing for the chemical attack in the Idlib province.
White House spokesman Sean Spicer told a briefing, "These heinous actions by the Bashar Al-Assad regime are a consequence of the last administration's weakness and irresolution. President Obama said in 2012 that he would establish a 'red line' against the use of chemical weapons and then did nothing."