US military action on Syria
Chairman of the Joint Chiefs General Martin E Dempsey (L), John Kerry, US Secretary of State (C) and Chuck Hagel, Secretary of Defence during a hearing on the Syrian crisis - Reuters Reuters

The head of America's armed forces suspects Russia may bolster its military supplies to the Assad regime if Barack Obama's proposed strike against Syria takes place.

General Martin Dempsey, Chairman of the US Joint Chiefs of Staff, told a Senate hearing there are signs of Moscow stepping up its aid if the US presses ahead with its attack.

"There is some indication that they [the Russians] have assured the [Assad] regime that if we destroy something, they can replace it," said Dempsey.

However Dempsey said this factor need not be a reason to delay military action against Syria, which has been gaining support from key senators.

Russia, Assad's staunchest ally, has publicly doubted the evidence provided by the US over the alleged Ghouta chemical attack on 21 August, and was the first to raise the alarm when Israel fired two ballistic missiles into the eastern part of the Mediterranean Sea.

The Russian navy has been heavily boosting its naval presence in the Mediterranean region over the last several months, and has moved a destroyer and advanced warships to the area.

Obama is headed to Sweden en route to Russia for the G20 summit, which is likely to be dominated by the Syrian crisis.

Republican and Democrat senators have given their support to a draft resolution pushing for military action. The draft prohibits the use of ground forces and limits the intervention to 60 days.

"I believe that my colleagues should support this call for action," John Boehner, Republican speaker of the House of Representatives, told reporters following the hearing.

Even though the proposal will be put to the vote next week when Congress reconvenes, the support appears to be a clear indication of the likely outcome.

Meanwhile, UN Secretary-General Ban Ki-moon said it would be illegal under international law for the US to launch a strike on Syria unless it is sanctioned by the Security Council sanction or is made in self-defence.