In a confident interview with US network Fox News, Assad insisted Syria was not gripped by civil war but was the victim of infiltration by foreign-backed al-Qaeda fighters.
His latest appearance came as UN envoys debated a draft resolution that would enshrine a joint US-Russian plan to secure and neutralise his banned weapons in international law.
The plan is to be discussed at a meeting in The Hague on Friday by the world's chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons.
It was his second interview this month with US television, and one of a series of meetings with Western journalists to counter mounting political pressure from Western capitals.
After last month's barrage of sarin-loaded rockets, which the West says was clearly launched by the regime, US President Barack Obama called for US-led punitive military strikes.
But with US lawmakers and the Western public not sold on the virtues of another Middle East military adventure, Assad's ally Russia seized the opportunity to propose a diplomatic solution.
Assad reiterated his pledge to cooperate, but insisted he had not been forced to do so by US threats of US action.
"I think it's a very complicated operation, technically. And it needs a lot of money, about a billion," he told Fox.
"So it depends, you have to ask the experts what they mean by quickly. It has a certain schedule. It needs a year, or maybe a little bit more."
"What we have is not civil war. What we have is war. It's a new kind of war," he said, alleging that Islamist guerrillas from more than 80 countries had joined the fight.
"We know that we have tens of thousands of jihadists... we are on the ground, we live in this country," he said, disputing an expert report that suggested 30,000 out of around 100,000 rebels were hardliners.
"What I can tell you is that... 80 to 90 percent of the underground terrorists are Al-Qaeda and their offshoots.
