US President Donald Trump condemned the "intolerable" alleged chemical attack on civilians in Syria and blamed the Bashar Al Assad regime, media reports said.

"Today's chemical attack in Syria against innocent people, including women and children, is reprehensible and cannot be ignored by the civilized world," Efe news quoted Trump as saying on Teusday in a statement.

Trump said that "heinous actions by the Bashar Al Assad regime are a consequence of the past administration's weakness and irresolution", referring to former President Barack Obama, who "said in 2012 that he would establish a 'red line' against the use of chemical weapons" in Syria but then "did nothing".

"The United States stands with our allies across the globe to condemn this intolerable attack," the President concluded.

Shortly before Trump issued his statement, a senior State Department official, who asked for anonymity, told reporters that everything pointed to the fact that the chemical attack was a "war crime" and that the countries backing Assad — Russia and Iran — have much to answer for.

US Secretary of State Rex Tillerson also had harsh words for Russia and Iran in his own statement condemning the attack, which killed at least 58 people in the town of Khan Sheikhoun.

"While we continue to monitor the terrible situation, it is clear that this is how Bashar al-Assad operates: with brutal, unabashed barbarism," said Tillerson in the statement.

Tillerson also said that the attack is the third complaint about the use of chemical weapons in Syria in the past month.

The US accusations come after Syria denied that either its forces or Russian forces were behind the attack.