The U.S. military and intelligence community captured communications between Syrian military officials and chemical experts getting ready for last week's chemical attack on civilians, CNN reports.

The communications were discovered in the hours following the attack that killed at least 70 people in the northwestern Syrian city of Idlib, CNN quoted a senior U.S. official as saying. The United States was not aware of the attack before it happened, the official stressed.

The information was part of a large-scale collection of data that usually is not analysed unless an event such as the sarin gas attack occurs.

Syrian President Bashar al-Assad has insisted his military did not use chemical weapons, but likely hit with conventional weapons a stockpile of chemical weapons stored by rebels.

The Trump administration has for days maintained Syria used chemical weapons and that its ally Russia likely was aware of such an attack before it happened.

But as yet no communications have turned up showing Russia was aware of the attack, and Moscow maintains it still believes the Assad line.