A civil defence member breathes through an oxygen mask, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. [Photo/Agencies]

Tillerson issued the call in a statement released after media reports said about 70 people were killed with 200 others wounded Tuesday in a gas attack in a rebel-held area in Syria's northwestern province of Idlib.

"The United States strongly condemns the chemical weapons attack in Idlib province, the third allegation of the use of such weapons in the past month alone," the top US diplomat said.

The United States calls upon Russia and Iran again "to exercise their influence over the Syrian regime and to guarantee that this sort of horrific attack never happens again," Tillerson said.

He added that Moscow and Tehran, as "the self-proclaimed guarantors to the ceasefire negotiated in Astana," bear great moral responsibility for these deaths.

Earlier, White House Spokesman Sean Spicer told in a briefing that the United States condemns the gas attack against innocent civilians as "heinous actions" by the Syrian government led by President Bashar al-Assad.

But he offered no proof to show that the Syrian government has carried out the attack.

The Syrian army on Tuesday refuted the accusations as completely baseless, saying it had not used, nor would use such weapons in the future. It also held "terrorist groups" responsible for using chemical weapons.

Earlier, the Syrian Observatory for Human Rights said warplanes launched intensive airstrikes on the rebel-held town of Khan Sheikhoun in Idlib, killing 58 people, mostly civilians.

The Syrian opposition accused the Syrian Air Force of backing the attack.

The attack was believed to be chemical and was launched by air, said the United Nations special envoy to Syria Staffan de Misturanoting, noting that there ought to be a "clear recognition of responsibility and accountability."