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.

It was the fear of militants coming to dominate the opposition that caused the United States and its Western allies to hold off providing lethal aid to the Syrian opposition, at least until now. But as a result, counterterrorism analysts say, they lost a chance to influence the battle in Syria. Even Congressional supporters of the C.I.A.’s covert program to arm moderate elements of the Syrian opposition fear the delivery of weapons, set to begin this month, will be too little, too late.

The stakes are high. American intelligence officials said this week that Ayman al-Zawahri, the overall leader of Al Qaeda in Pakistan, has had regular communications with the Nusra Front in Syria, reflecting how favorably the Qaeda leadership views the long-term potential for Syria as a safe haven. Juan Zarate, a former senior counterterrorism official in the George W. Bush administration, said that Syria lay in the center of an arc of instability, stretching from Iran through North Africa, and “in that zone, you may have the regeneration and resurrection of a new brand of Al Qaeda.”

In Syria, the battle lines have hardened in recent months. The Syrian government, backed by Iran and Hezbollah, has seized new momentum and retaken territory in the south and east from the rebels. At the same time, power within the badly fractured opposition, numbering about 1,200 groups, has steadily slipped into the hands of the jihadists based in the northeast, where this week they seized a strategic airport in the area. They also hold sway in the provincial capital of Raqqa.

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.

Mr. Assad’s argument “began as a fiction during the period of peaceful, unarmed protests but is now a reality” because of Mr. Assad’s own efforts to divide the country as well as the success of the extremists, Hussein Ibish, a senior fellow at the American Task Force on Palestine, wrote in a recent essay that appeared in The National.

In Raqqa recently, a commander of the Islamic State in Iraq and Syria sipped coffee after breaking the Ramadan fast, wearing a Pakistani-style outfit. The commander, Abu Omar, was Syrian, a member of a tribe in the area, but he described his movement’s goals as reaching far beyond the country’s borders.

He did not speak of attacking the United States. But he threatened Russia, and he spoke of a broad-based battle against Shiite-led Iran and its quest to dominate the region, and said Sunnis from across the world were justified in flocking to Syria to fight because of the government’s reliance on Shiite fighters from Lebanon and Iraq.

Anne Barnard reported from Beirut, and Eric Schmitt from Washington. An employee of The New York Times contributed reporting from Raqqa, Syria.