The United States, Turkey and other Western states have blamed the regime of Syrian President Bashar al-Assad for the deaths. The Assad government used chlorine in the past, while ISIS terrorists attacked with sarin and mustard gas.
"We don't know whether those dead children were killed in Khan Sheikhun".
"This story is not convincing by any means".
Meanwhile, a senior USA official who spoke under condition of anonymity said the US found that Russian Federation already knew of the chemical attack before it was carried out.
"We can only allow any investigation when it's impartial, when we make sure that unbiased countries will participate in this delegation to make sure that they won't use it for politicised purposes", he said.
"The United States is not serious in achieving any political solution".
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.
The OPCW based in Hague sent their team to collect biometric samples and to interview survivors.
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"Our impression is that the West, mainly the United States, is hand-in-glove with the terrorists", Assad told Agence France-Presse in his first comments since a USA missile strike on a Syrian air base in retaliation for the chemical attack.
The Syrian army on Thursday said the US -led anti-terror coalition struck a chemical weapons depot controlled by the Islamic State (IS) group in eastern Syria, killing hundreds, including civilians.
A closer look at photos from the town of Khan Shaykhun shows that the chemical attack site was tampered with and that the USA report blaming the Syrian government can't be true, says the MIT professor skeptical of the White House narrative.
"This incident is a sign of the coordination between the terrorist groups and the powers supporting them to find pretexts to frame the Syrian army and accuse it of using chemical weapons", the statement said, 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.
Earlier testing by Turkish authorities had also said the chemical used on April 4 was sarin.
The OPCW mission will determine whether chemical weapons were used, but is not mandated to assign blame. 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.
Investigators determined in the past that during Syria's civil war, sarin, chlorine and mustard gas were used.