In an exclusive interview with AFP in Damascus - his first since the alleged April 4 attack prompted a U.S. air strike on Syrian forces - Assad said his army had given up all its chemical weapons and that Syrian military firepower was not affected by the United States strike.
Asked if airstrikes by Syrian troops could have struck a terrorist chemical weapons depot, Assad declined to speculate, saying it was often hard to know the details about targets from aerial pictures.
Assad gave the interview to AFP in Damascus on Wednesday.
Assad said he would only allow an "impartial" external investigation involving "unbiased countries", according to AFP.
Western powers blame the gas attack, which killed scores of civilians - many of them children - on Assad's forces.
Trump said it's "certainly possible" that Russian Federation had advance knowledge of the use of chemical weapons by its ally, Assad's regime.
A man breathes through an oxygen mask as another one receives treatments, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017.
It said Mansour admitted to fighting against Lebanese troops in northern Lebanon in the past and was involved in smuggling weapons to the al-Qaida-linked Nusra Front in Syria, now known as the Fatah al-Sham Front.
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While most persecution watchdog groups, including Open Doors USA, declined to comment on whether Trump is doing the right thing by bombing Syrian government targets, some, such as Oxfam America, questioned the US president's motives.
However, Washington came to a conclusion that Damascus had used chemical weapons which led the USA to carry out a missile attack on a Syrian military air base located in the Homs Governorate.
The tests were not part of an official investigation launched by the Organization for the Prohibition of Chemical Weapons in the wake of the attack.
"Our firepower, our ability to attack the terrorists hasn't been affected by this strike".
Russian forces present at the base were given advance warning of the strike, which means it caused limited damage to Russian and Syrian air force capabilities despite its ferocity.
The OPCW has blamed Assad's government for at least two attacks in 2014 and 2015 involving the use of chlorine.
There's also video and satellite imagery suggesting the attack hit "in the middle of a street", not a storage depot, the NSC report says.
The OPCW's Fact Finding Mission for Syria is conducting an investigation and is expected to report its findings in three weeks.
"We are deeply anxious by our partners in the U.N. Security Council trying to evade an honest investigation into that episode", he said.