Syria's President Bashar al-Assad speaks during an interview with Croatian newspaper Vecernji List in Damascus, Syria, in this handout picture provided by SANA on April 6.
US Defense Secretary James Mattis at the time said the US cruise missile strike destroyed over 20 Syrian warplanes, some 20 percent of the regime's working aircraft.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said its tests on victims of the incident were "incontrovertible" proof sarin or a sarin-like substance was used.
Assad claimed the toxic gas attack had been faked by the rebels and the West to justify the Shayrat airbase strike. Despite rising tensions with Russian Federation, the United States remains committed to preventing further chemical attacks by the Syrian government. Reminding of then-US Secretary of State Colin Powell's white powder vial at the United Nations and other allegations of the chemical weapons "threat" in Iraq it has said that special equipment to collect samples for further scientific analysis should be used at the airfield, adding that Russian Federation has already undertaken similar procedures in Aleppo, Syria where it said chemical weapons had been used by militants.
Last week Mattis reiterated warnings that further chemical weapons use by Assad will be met with fresh USA action.
Assad has denied the allegations that he was behind the April 4 attack in the opposition-held town of Khan Sheikhun in Syria's southern Idlib province.
CNN bases its report on comments by two US defense officials.
After all, Russian Federation doesn't want a military confrontation with the Trump administration, and it can not be confident that Trump won't respond to another chemical attack by going after Assad's planes wherever they are housed.
Without any investigation carried out, the U.S. labeled Assads government as perpetrators and fired a barrage of Tomahawk missiles at Shayrat airbase, which it said was the source of the attack.
A USA official says that Syria has moved all of the aircraft from the airbase that was struck by US missiles two weeks ago to an airbase used by Russian Federation for missions within Syria.
"It could also be used for roughly 10 attacks of a similar size to the recent Khan Sheikhoun attack", he said.
Aleppo was divided between government and rebel-held districts for years, but government forces managed to drive rebels from the city in December with a Russian-backed offensive.
David Cameron had previously lost a Commons vote by 285 to 272 for military action against the Syrian government in the wake of a sarin attack that left hundreds of civilians dead on the outskirts of Damascus.
"The bottom line is there can be no doubt in the global community's mind that Syria has retained chemical weapons in violation of its agreement and its statement that it had removed them all", Mr Mattis said.
Earlier this week, Assad's former chemical-weapons research chief told Britain's The Telegraph that Syria had "at least 2,000 tons" of chemical weapons before the war and declared only 1,300.