France will step up aid to Syria's opposition if the United States Congress votes against a military strike to punish Syria's president for carrying out a reported chemical attack, President Francois Hollande said on Tuesday.

Hollande has been one of the main international voices calling for military action against Syria over the chemical attack, which he and Washington say killed thousands of people in Damascus on August 21.

But he was left out on a limb after Britain's parliament voted against taking part in any action and U.S. President Barack Obama said he would seek Congress approval before any assault.