Overt support
Separately from the president's secret order, the Obama administration has stated publicly that it is providing some backing for Assad's opponents.
The State Department said on Wednesday the US government had set aside a total of $25 million for "non-lethal" assistance to the Syrian opposition. A US official said that was mostly for communications equipment, including encrypted radios. .
The State Department also says the United States has set aside $64 million in humanitarian assistance for the Syrian people, including contributions to the World Food Program, the International Committee of the Red Cross and other aid agencies.
Also on Wednesday, the US Treasury confirmed it had granted authorization to the Syrian Support Group, Washington representative of one of the most active rebel factions, the Free Syrian Army, to conduct financial transactions on the rebel group's behalf. The authorization was first reported on Friday by Al-Monitor, a Middle East news and commentary website.
Last year, when rebels began organizing themselves to challenge the rule of Libyan leader Muammar Gaddafi, Obama also signed an initial "finding" broadly authorizing secret US backing for them. But the president moved cautiously in authorizing specific measures to support them.
Some US lawmakers, such as Republican Senators John McCain and Lindsey Graham, have criticized Obama for moving too slowly to assist the rebels and have suggested the US government become directly involved in arming Assad's opponents.
Other lawmakers have suggested caution, saying too little is known about the many rebel groups.
Recent news reports from the region have suggested that the influence and numbers of Islamist militants, some of them connected to al Qaeda or its affiliates, have been growing among Assad's opponents.
US and European officials say that, so far, intelligence agencies do not believe the militants' role in the anti-Assad opposition is dominant.
While US and allied government experts believe that the Syrian rebels have been making some progress against Assad's forces lately, most believe the conflict is nowhere near resolution, and could go on for years.