"Failure awaits the United States as in all previous wars it has unleashed, starting with Vietnam and up to the present day," Assad told Russian newspaper Izvestia on Monday when asked what would happen if Washington decided to strike or invade Syria.
Assad said Syrian government forces had been close to where rebel forces say chemical weapons were used last week during the country's more than two-year civil war.
"Would any state use chemical or any other weapons of mass destruction in a place where its own forces are concentrated? That would go against elementary logic," Assad told Izvestia, the newspaper.
Russia has been Assad's most important international ally throughout the civil war, supplying his troops with arms and resisting pressure at the United Nations for tighter sanctions on Damascus.
Asked about the arms deliveries, Assad said: "I want to say that all contracts that have been concluded with Russia are being fulfilled."
He gave no details and did not say whether Damascus had taken delivery of advanced S-300 sir defence systems from Russia which could vastly enhance its defence capabilities.
Washington's possible military action
Meanwhile, the US on Sunday said there was "very little doubt" Syrian forces had used chemical weapons on civilians, and dismissed an offer by Damascus for a UN team to view the attack site.
The comments marked a significant escalation of a showdown over the horrific attack outside the Syrian capital that killed up to 1 300 people last week, and came as Washington appeared to be positioning for possible military action.
Officials said US President Barack Obama, who held crisis talks on Saturday with top aides, would make an "informed decision" about how to respond to an "indiscriminate" chemical weapons attack.
An official told Agence France-Presse that based on the reported number of victims and their symptoms, and US and foreign intelligence, "there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident".
The official, speaking on condition of anonymity, said that Washington had noted that Syria had offered to let UN inspectors view the site of the alleged attack on Monday, but said it was too little, too late, and that the evidence available at the site "has been significantly corrupted" due to the delay.
A US diplomatic offensive led by Secretary of State John Kerry, comments coming from the White House and signs the Pentagon is positioning ships closer to Syria fuelled an impression that Obama may be getting ready to jettison his antipathy to new Middle Eastern entanglements and to order limited military action.
'The responsible way forward'
Kerry has spent days on the phone with Washington's foreign partners.
On Sunday Kerry spoke with UN Secretary General Ban Ki-moon, as well as his British, French, Canadian and Russian counterparts, a senior state department official said, speaking on condition of anonymity.
In all of the calls, Kerry "stressed that if the Syrian regime wanted to prove to the world that it had not used chemical weapons in this incident, it would have stopped shelling the area and granted immediate access five days ago", the official said.
Kerry "made clear that … there is very little doubt that a chemical weapon was used by the Syrian regime against civilians in this incident", based on intelligence reports, and information from US international partners.
Kerry also reiterated that Obama "is studying the facts and will be making an informed decision about the responsible way forward", the official said. – Reuters; AFP