"If the Americans, French or British have evidence about the alleged chemical weapon attack, they would have declared it," Xinhua quoted the president as telling French daily Le Figaro in an interview.
The remarks came amid US threats to carry out a military operation against Syria over the alleged use of chemical weapons against Syrian people in the countryside of Damascus Aug 21.
"Whoever accuses the Syrian army of using chemical weapons should be responsible to show his evidence. We have challenged them to provide one evidence to substantiate their accusation, but they did not, and will not do. If they were in possession of evidence, they would have already shown it," Assad said.
"Strong is the one who prevents war not the one who ignites it," the Syrian president said.
"If the US wages an aggression against Syria, everyone will lose control and the result will be the spread of violence, war and extremism because the situation is not only related to Syria, but the entire region," Assad said.
"What benefit will the world achieve from supporting the terrorism in Syria?" the president questioned.
On US President Barack Obama's recent tactic to seek Congressional approval for a strike on Syria, Assad said whoever wants to vote must ask himself -- what good did previous wars bring to America and Europe? What result did the world achieve after the war on Libya and Iraq?
Any congressman must work for the interest of his country. If they think logically, they would not see any interest to America in authorizing such a war, he said.
On the situation in the region, Assad said: "The Middle East is a powder keg and the fire has become so close to this keg. The matter is related not only to the Syrian response (to the possible US war) but to what will happen after the first strike?"
"Whoever puts the plan of war can tell you about the first step only... but later no one can tell what will happen... Everybody could lose control and the keg would explode."
France will also face repercussions if Paris and its allies launch a military intervention against his country.
"There will be repercussions, negative ones obviously, on French interests," the Syrian president told the French Daily.
"Anybody who contributes to the financial and military reinforcement of terrorists is the enemy of the Syrian people. The French are not our enemy... but if the policies of the French state are hostile to the Syrian people, the state will be their enemy," said Assad.
"Chaos and extremism would ensue and there is a risk of regional war," he warned.
The remarks also came as the White House geared up efforts to win Congressional support for a strike on Syria.
On Monday, Obama invited Republican senators John McCain and Lindsey Graham to the White House to discuss military intervention in Syria.
Obama has announced he would seek authorisation by Congress for military action over the alleged Aug 21 chemical weapons attack in the suburbs of Damascus, which reportedly killed at least 1,429 people, including 426 children. The Syrian government has denied the accusation.