BEIRUT, Lebanon — As foreign fighters pour into Syria at an increasing clip, extremist groups are carving out pockets of territory that are becoming havens for Islamist militants, posing what United States and Western intelligence officials say may be developing into one of the biggest terrorist threats in the world today.

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.