Washington, April 13 The US military and intelligence community has intercepted communications featuring Syrian military and chemical experts talking about preparations for the April 3 attack that killed over 80 civilians, a senior US official told CNN.

The intercepts were part of an immediate review of all intelligence in the hours after the attack in Idlib.

"The US did not know prior to the attack it was going to happen," the official said late Wednesday night.

The US scoops up such a large volume of communications intercepts in areas like Syria and Iraq, the material often is not processed unless there is a particular event that requires analysts to go back and look for supporting intelligence material.

So far there are no intelligence intercepts that have been found directly confirming that Russian military or intelligence officials communicated about the attack.

The official said the likelihood is the "Russians are more careful in their communications to avoid being intercepted", reports CNN.

The Russian and Syrian governments have both denied involvement in the chemical attack.

US President Donald Trump told a White House news conference on Wednesday that the Pentagon is looking into the question of Russian complicity in the chemical attack.

"I would like to think that they didn't know, but certainly they could have," CNN quoted Trump as saying.

"(Defense Secretary) General (James) Mattis is looking into it with the entire Pentagon group that does that kind of work."

The US now assesses that Syria has re-established a unit of personnel associated with chemical weapons that existed before the 2013 agreement in which the Syrian government pledged to give up its weapons inventory.

\'US intercepted communications between Syrian military, chemical experts\'