Syrian army's statement came nearly a week after the opposition accused the Syrian government air force of firing toxic gas on the rebel-held town of Khan Sheikhoun in Idlib province.
Postol further noted in his report, "I believe it can be shown, without doubt, that the document does not provide any evidence whatsoever that the USA government has concrete knowledge that the government of Syria was the source of the chemical attack in Khan Shaykhun".
Assad insisted it was "not clear whether it happened or not, because how can you verify a video?".
Tests have confirmed sarin gas was used in Khan Sheikhoun, Syria. The Britain-based Syrian Observatory for Human Rights monitoring group reported 25 SDF fighters were killed in the last two days of battle, while the activist-run Raqqa is Being Slaughtered Silently group said on Wednesday that three days of mourning were to be held in a border town held by the SDF after the arrival of the bodies of 15 fighters from the battle. He said the strike "opens new disturbing scenarios for all", considering that there was no thorough investigation into the chemical attack.
Assad said he would only allow an "impartial" external investigation of last week's suspected chemical attack. "These have tested positive for the nerve agent Sarin, or a Sarin-like substance", the delegation said during a special session at the Organisation for the Prohibition of Chemical Weapons in The Hague.
Trump says US-Russia relations may be at "all-time low"
The Kremlin had previously declined to confirm Putin would meet Tillerson, reflecting tensions over the USA strike on Syria. One of Mr Lavrov's deputies was even more scathing.
The toxic gas attack, which killed scores of people including children, prompted a United States cruise missile strike on a Syrian air base and widened a rift between the USA and Russian Federation, a close ally of Assad's in his conflict with rebels and militants.
"The United Kingdom therefore shares the U.S. assessment that it is highly likely that the regime was responsible for a sarin attack on Khan Sheikhun on the 4th of April", Ambassador Matthew Rycroft told the UN Security Council.
Meanwhile, the Syrian government has fiercely denied using or even possessing chemical weapons since the country's compliance with the Chemical Weapons Convention was certified by worldwide observers in 2013. The American leader also announced a missile strike targeting an air base believed to be the source of the chemical weapons used in the attack. The delegation's mission is to unequivocally confirm whether or not chemical weapons were utilized without blaming any specific entity. Its findings, expected in 3-4 weeks, will be passed to a joint United Nations-OPCW investigation tasked with identifying individuals or institutions responsible for using chemical weapons.
Global investigators have concluded that sarin, chlorine and sulphur mustard gas have been used in Syria's 6-year-old conflict, with government forces using chlorine and Islamic State militants using sulphur mustard.