A top US Senate Democrat introduced a bill Monday that authorizes arming rebels in Syria, a step Washington has been weighing after President Barack Obama said the Damascus regime may have used chemical weapons.

Senator Robert Menendez, chairman of the Senate Foreign Relations Committee, introduced the Syria Stabilization Act of 2013 that would give "increased authorization to provide lethal and non-lethal assistance to Syrian armed opposition."

The bill is seen as intensifying pressure on Obama to take action in Syria after US and other intelligence assessments that the regime has used chemical weapons, something Obama has warned would be a "game changer."

"The Assad regime has crossed a red line that forces us to consider all options," Menendez said in a statement.

"The greatest humanitarian crisis in the world is unfolding in and around Syria, and the US must play a role in tipping the scales toward opposition groups and working to build a free Syria."

The measure specifically bars the administration from transferring portable, shoulder-fired missile or grenade launchers known as MANPADS, amid concern that such weapons have been finding their way from places like Libya into the hands of extremists who might use them against US interests.

It also increases sanctions on arms and oil sales to strongman Bashar al-Assad's regime, and authorizes a "transition fund" of some $250 million per year aimed at helping a civilian opposition in Syria prepare for a switch to some form of democratic rule.

The bill is likely to garner some bipartisan support, as a handful of Republicans including Senator John McCain have long said it was time to consider arming vetted opposition groups.

The Obama administration has expressed worry about the risks of pouring more arms into a volatile conflict, and has so far stuck to providing humanitarian assistance and non-lethal aid.

But Defense Secretary Chuck Hagel acknowledged last Thursday that Washington was taking a fresh look at whether to arm Syria's outgunned rebels.