The attack on April 4 killed more than 80 people.
U.S. officials were the first to say they believed Sarin gas was used in the attack, but they offered no concrete evidence.
He added that the Syrian army had given up all its chemical weapons and that Syrian military power was not affected by the US strike.
President Bashar Assad said a chemical attack on a rebel-held town earlier this month that was widely blamed on his forces was a "fabrication".
A United Nations Security Council resolution condemning the use of chemical weapons in Syria was vetoed by Russian Federation on Wednesday - the resolution, drafted by the US, UK and France, included a disputed paragraph that demanded investigators be given immediate access to the site.
Mr Assad told AFP that he would only allow an "impartial" investigation, involving "unbiased countries.to make sure that they won't use it for politicised purposes".
"Definitely, 100 percent for us, it's fabrication", Assad said in the video released by his office.
He added: "We don't know whether those dead children were killed in Khan Sheikhoun". "Were they dead at all".
The US response - 59 Tomahawk missiles fired the airbase from which the chemical-weapons attack was supposedly launched - appears to signal a shift in policy under President Trump, who said that it was "in the vital national security interest of the US to prevent and deter the use of deadly chemical weapons".
Turkey's Erdogan Accuses West of Campaigning Against Him in Referendum
Turkish voters are to decide on Sunday whether to grant President Recep Tayyip Erdogan expanded powers in a referendum . Polls suggest a neck-and-neck race for Sunday's vote. "A new referendum could also be discussed".
More than 300,000 people have lost their lives and millions of people have been displaced since a peaceful uprising against Mr Assad six years ago turned into a full-scale civil war.
A senior Russian diplomat says the United States and Russia are again exchanging information on a hotline created to prevent midair incidents over Syria, but that the formal agreement on such exchanges has not been renewed.
In his AFP interview, Assad accused the West, particularly the United States, of being "hand-in-glove with the terrorists" over the Khan Sheikhoun incident. He said the Syrian people "have suffered six years of ever-escalating barbarity, over six years of failed ceasefires, and over the years, Russian Federation has held us ransom for the Assad regime". The retaliatory strike was praised by many world leaders, while Russian Federation denounced it as an act of "aggression" and accused the USA of violating worldwide law. "At the right time everyone will come to their senses & there will be lasting peace!"
The prime minister said the foreign secretary did "an excellent job in bringing together the G7".
On Thursday, Moscow hosted Syrian Foreign Minister Walid Muallem for talks with Lavrov, ahead of a three-way meeting Friday with Foreign Minister Mohammad Javad Zarif of Iran, another Assad ally.
Assad said no proper investigation had been possible because the area was controlled by Nusra Front.
The UK and the U.S. are united in their criticism of Russia's support for the Syrian president.
The coalition said Thursday one its air strikes had accidentally killed 18 members of the Syrian alliance south of Tabqa, a strategically important town on the Euphrates River near Raqa.
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