The OPCW announced in January past year that it had destroyed Syria's chemical weapons arsenal per an agreement of the Syrian Government.
The interview with Assad was broadcast as the Syrian government asserted without evidence that USA warplanes had bombed what it called a chemical weapons cache possessed by Islamic State group militants in Syria on Wednesday, releasing poisons that it said had killed hundreds.
Assad said more USA attacks "could happen anytime, anywhere, not only in Syria".
But the UK Ministry of Defence and Turkish authorities say they have carried out tests on samples which provide evidence that Sarin or a similar substance was used.
Turkey and the United Kingdom say tests show Sarin or a Sarin-like substance was used in Khan Sheikhoun, which would be the first time since 2013 that a prohibited chemical had been used on such a scale.
United States officials were the first to say they believed Sarin gas was used in the attack, but they offered no concrete evidence. The health minister in Turkey, which treated numerous attack's victims and conducted autopsies on others, said Tuesday that test results conducted on victims confirmed sarin gas was used.
The suspected chemical attack killed at least 87 people, including many children, and images of the dead and of suffering victims provoked global outrage. "You have a lot of fake videos now", he says. It's not clear whether it happened or not, how can you verify a video?
More than 80 people were killed in what the USA has described as a nerve gas attack that Assad's forces undoubtedly carried out.
Could Syria have launched the chemical weapons attack with Russia's advance knowledge?
"We both have agreed our lines of communication shall always remain open", Tillerson said. "In his Wednesday interview with AFP, Assad similarly said his nation would allow only an "impartial" investigation".
Syria and Russian Federation have suggested a conventional airstrike by the Syrian army may have hit a depot storing chemical weapons possessed by extremists. Those affected by the attack were taken to nearby hospitals.
Both Syria and Russian Federation, a longtime ally of Assad in the nation's ongoing civil war, have denied involvement in the chemical strike and have sought to place blame for the attack on terrorist groups in the region looking to spur US military action against Assad.
According to Assad, the Syrian military dismantled all of its chemical weapons arsenal in 2013.
He said that India believed that it was important to defeat terrorism "which in that particular theatre is represented, among others by the IS (Islamic State) and cooperation of all countries is vital to achieving this objective".
In the lead up to the government takeover of the rebel-held half of Aleppo last past year, Human Rights Watch reported on the government's "systematic" use of chlorine gas, marking at least eight incidents in which military helicopters dropped the chemical into residential areas.
Meanwhile, the Syrian army said hundreds of ISIL fighters as well as civilians were killed when an air raid by the US-led coalition hit a militant position in the eastern province of Deir Ezzour.