Syrian President Bashar Assad dismissed allegations that his forces have used chemical weapons in an interview published by a Russian newspaper on Monday and warned Washington that any U.S. military intervention would fail.

"Failure awaits the United States as in all previous wars it has unleashed, starting with Vietnam and up to the present day," he told the Izvestia daily 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-old 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, a pro-Kremlin 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 defense systems from Russia which could vastly enhance its defense capabilities.

Russia has expressed its concern to Washington that the United States will respond militarily to a suspected chemical weapons attack by Syrian government forces and urged restraint, Russia's Foreign Ministry said on Monday.

Referring to a telephone conversation between Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry on Sunday, the ministry said Moscow had also urged Washington to refrain from falling for "provocations".

"The minister (Lavrov) stressed that the official announcements from Washington in recent days about the readiness of U.S. armed forces to 'intervene' in the Syrian conflict have been received in Moscow with deep concern," the ministry said in a statement.

U.S. remarks that Syria's agreement to allow the United Nations to inspect the site of the suspected chemical weapons attack was "too late to be credible" appeared to signal a military response was more likely.

A senior senator said he believed President Barack Obama would ask for authorization to use force when Congress returned from recess next month.

But Russia, an ally of Assad, has suggested rebels may have been behind the alleged chemical weapons attack.

"In connection with this, the Russian side calls for (Washington to) refrain from the threat of force on Damascus, to not fall for provocations and to try to help create normal conditions to give the U.N. chemical experts' mission, which is already in the country, the possibility of conducting a thorough, objective and impartial investigation," the statement said.