In an interview with CBS News, the Syrian leader also said that he would be "open to a dialogue with the United States, but that it must be "based on mutual respect." CBS has published excerpts of the interview online.
The Syrian opposition and activists say government helicopters dropped bombs containing chlorine gas on the town of Sarmin in northwestern Syria's Idlib province on March 16, killing six people.
Videos posted online showed people struggling to breathe, and the international humanitarian organization Doctors Without Borders said symptoms described by medics in contact with the group clearly indicate the presence of chlorine poisoning.
"It's not used as military gas. That's very self-evident.
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Traditional arms is more important than chlorine, and if it was very effective, the terrorists would have used it on a larger scale," he said. The government refers to its opponents as "terrorists."
The purported attack on Sarmin would be one of the most serious uses of poison gas in Syria since a deadly chemical attack outside Damascus in August 2013. In the fallout from that attack, Assad relinquished his chemical weapons program under a US-Russian brokered deal, averting threatened American military strikes.
The Organization for the Prohibition of Chemical Weapons OPCW has concluded "with a high degree of confidence" that chlorine was used on three rebel-held villages in Syria last year, killing 13 people. It did not assign blame. Last month, the OPCW condemned the use of chlorine in Syria as a breach of international law.
In the interview, Assad also addressed US Secretary of State John Kerry's recent statement that Washington must eventually talk with Damascus to help negotiate an end to Syria's civil war.