"There’s no doubt in our mind that the Syrian regime under the leadership of Bashar al-Assad is responsible for this horrific attack," Tillerson said during a press conference at the White House alongside Mexico’s Foreign Secretary Luis Videgaray Caso.

"And we think it’s time that the Russians really need to think carefully about their continued support of the Assad regime," he added.

Tillerson will visit Moscow on April 12.

The attack Tuesday on the town of Khan Shaykhun killed at least 100 victims and injured more than 500 others, according to the Syrian opposition's health minister Firas Jundi.

During a UN Security Council meeting Wednesday, U.S. Ambassador Nikki Haley suggested Washington would take unilateral action if Russia fails to help the international community eradicate chemical weapons use.

"When the UN consistently fails in its duty to act collectively, there are times in the life of states when we are compelled to take our own action," Haley said.

President Donald Trump later said the attack "crosses many, many lines” and that the deaths are "an affront to humanity".

Anadolu Agency