Known as fierce fighters willing to employ suicide car bombs, the jihadist groups now include more than 6,000 foreigners, counterterrorism officials say, adding that such fighters are streaming into Syria in greater numbers than went into Iraq at the height of the insurgency there against the American occupation.

Many of the militants are part of the Nusra Front, an extremist group whose fighters have gained a reputation over the past several months as some of the most effective in the opposition.

But others are assembling under a new, even more extreme umbrella group, the Islamic State in Iraq and Syria, that is merging some Syrians with fighters from around the world — Chechnya, Pakistan, Egypt and the West, as well as Al Qaeda in Iraq, the Sunni insurgent group that rose to prominence in the fight against the American occupation in the years after the 2003 invasion. The concern is that a new affiliate of Al Qaeda could be emerging from those groups…

The idea that Syria could supplant Pakistan as the primary haven for Al Qaeda someday, should the government fall, is a heavy blow to the Western-backed Syrian opposition and its military arm, the Free Syrian Army. It plays directly into the hands of Syria’s president, Bashar al-Assad, whose government has sought to portray itself as the only alternative to Islamic extremism and chaos, and has made the prospect of full-on American support even more remote than it already was.