President Obama reportedly has signed an intelligence finding that authorizes U.S. support for Syrian rebels seeking to overthrow President Bashar al-Assad’s dictatorship.  It remains unclear what is included in this “covert” aid, publicized in what looks like another leak, or when the finding was signed.

But the United States must be very careful in picking Syrian opposition groups to support.  As Syria melts down in an intensifying civil war, al-Qaeda’s black flags have been planted in growing numbers inside Syria.

Islamist extremists in eastern Syria, many of whom had flocked to fight U.S. troops inside Iraq or provided logistical support to Al-Qaeda in Iraq, have been exercising a growing influence inside the armed opposition to the Assad regime. Many foreign militants, including radical Libyan Islamists, have also joined the fighting in Syria. The Guardian reported the unease of one Syrian rebel who noted that the al-Qaeda-affiliated groups often have greater financial support, better weapons, and more military expertise than the Free Syrian Army (FSA), the umbrella group that links many of the local rebel forces:

“They have better financing than the FSA and we have to admit they are here.… They are stealing the revolution from us and they are working for the day that comes after.”

Concern about what happens “the day after” is a major reason that the United States should be extremely careful about how it helps Syria’s splintered opposition. Washington should calibrate its efforts to bring down Bashar al-Assad’s dictatorship in a manner that does not inadvertently contribute to the rise of an Islamist extremist dictatorship that becomes a base for international terrorism.

The U.S. has so far provided non-lethal supplies, such as communications gear, to the opposition but apparently has refrained from transferring arms. But Syria’s opposition forces have obtained a growing arsenal of weapons from the black market, defected or captured soldiers, and foreign sources. Rebels in northern Syria have reportedly acquired nearly two dozen MANPADS (man-portable air defense systems) that were transferred to them in Turkey, probably supplied by Saudi Arabia or Qatar, the leading Arab supporters of the Syrian opposition.

The MANPADS, shoulder-fired anti-aircraft missiles, will undoubtedly come in handy in countering the Assad’s regime’s increased use of fixed-wing aircraft and helicopters, particularly in Aleppo, where the rebels have staged an offensive to gain control of Syria’s largest city.

If the opposition can consolidate its control over Aleppo, Syria’s commercial hub, then Syria could reach an important turning point. But the question of what form of government ultimately replaces the hated Assad regime remains very much in doubt.