The Syrian leader also questioned talks to be held in Moscow this week, telling Foreign Affairs magazine that his government would attend but was not convinced the opposition figures taking part represented Syrians on the ground.
Washington has backed the Syrian opposition since early in the uprising and has unveiled plans to train more than 5,000 vetted rebels in Qatar, Saudi Arabia and Turkey to fight IS.
Assad said the planned US-trained force would be "illegal" and would be treated like any other rebel group.
"They are going to be fought like any other illegal militia fighting against the Syrian army," he said.
"Bringing 5,000 (fighters) from the outside will make most of them defect and join ISIS (Islamic State) and other groups.
"The idea itself... is illusory."
The Pentagon has itself acknowledged that identifying and vetting potential rebel recruits for training is a difficult task that cannot be accomplished quickly without significant risks.
Assad questioned the seriousness of the US-led campaign against the jihadists.
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"What we've seen so far is just, let's say, window-dressing, nothing real," he said.
"Did the United States put any pressure on Turkey to stop the support of Al-Qaeda? They didn't," Assad said.
He was referring to his government's longstanding accusations that Ankara has backed rebel groups including IS's jihadist rivals in Al-Qaeda affiliate Al-Nusra Front.
Assad said the nearly four-year-old conflict could only be ended with a political solution, but cast doubt on the value of talks being organised this week by his key ally Russia.
The dialogue, which was due to open later on Monday, was intended to bring together government and opposition representatives, but the main exiled opposition bloc, the National Coalition, is boycotting.
Assad said his government would attend, but asked: "Who do you negotiate with?
"We have institutions, we have an army and we have influence," he said.
"The people we are going to negotiate with, who do they represent?"
His government has long argued that the exiled opposition does not represent people inside Syria, accusing it of being "puppets" of its main foreign backers, Qatar, Saudi Arabia, Turkey and the United States.