By David Ignatius - May 14, 2014

WASHINGTON -- The chamber of horrors of the Syrian civil war has spawned a terrorist group so extreme that it has been rejected even by al-Qaeda -- and this toxic group is now establishing a safe haven in the city of Raqqah in northern Syria that could soon be used to attack foreign targets.

The hyper-militant terrorist group is known in the West as the Islamic State of Iraq and Syria, or ISIS. Its efforts to establish a Muslim caliphate that spans the two countries, and to absorb another al-Qaeda offshoot known as the al-Nusra Front, brought a rebuke a year ago from al-Qaeda's chief, Ayman al-Zawahiri.

"I have to choose between the rule of God and the rule of Zawahiri, and I choose the rule of God," thundered Abu Bakr al-Baghdadi, the charismatic leader of ISIS. The group went its own fiery way after it was denounced by Zawahiri, leaving the al-Nusra Front as al-Qaeda's official Syrian affiliate.

The terrorist threat in Syria is germinating in the dark space between the weak moderate opposition movement, which has been pleading for more Western training and assistance, and the embattled regime of President Bashar al-Assad. For now, the main check on ISIS' growth is that it burns so hot, and operates so brutally, that it alienates people where it takes root.

U.S. intelligence agencies are working with counterparts in the Middle East and Europe to track the ISIS and al-Nusra Front operatives, and to monitor foreign fighters who have traveled to Syria to join the jihad. This effort to map the Sunni jihadist networks has drawn a useful portrait:

-- Of the roughly 110,000 total opposition fighters in Syria, ISIS accounts for somewhere between 5,000 and 10,000; the al-Nusra Front has 5,000 to 6,000 committed fighters. A third militant Sunni group called Ahrar al-Sham has 10,000 to 15,000 members, some of whom have extremist leanings. The extremists in these groups are regarded as the toughest and most motivated warriors within the opposition.

-- About 10,000 to 15,000 foreigners have joined the opposition, traveling from such diverse locations as Chechnya, Australia, Libya, Belgium and the United States. Intelligence analysts are said to be especially concerned about roughly 1,500 foreign fighters who hold European passports, which allow them to travel freely across the continent and to enter the U.S. with relative ease.

U.S. officials believe that ISIS is providing tactical expertise and training facilities to these foreign fighters -- building the infrastructure for foreign terrorist operations. Moreover, Baghdadi and other top leaders are said to have issued multiple statements over the past two years threatening international attacks.

In the view of these officials, ISIS could attempt an attack outside the Middle East soon, and several plots have already been disrupted. But analysts believe that the group is focused now mostly on battles within Syria and Iraq.

The group has established a secure haven in Raqqah, a city of about 220,000. Its fighters control the roads in and out. They sell the region's oil and natural gas resources to finance their operations, supplementing revenues from kidnapping and other criminal activities. U.S. officials believe ISIS is now self-financing and no longer needs donations from wealthy supporters in the Gulf.

As its name implies, ISIS seeks to mobilize Sunnis in both Iraq and Syria. The Iraq branch is battle-hardened from its years of fighting under Abu Musab al-Zarqawi against U.S. military occupation. Their Iraq force is smaller than in Syria, probably numbering in the hundreds, but it has a tight command structure and has spearheaded Sunni attacks against government troops in Fallujah and other neighborhoods west of Baghdad.

U.S. officials view Baghdadi as a dynamic leader who mobilizes Iraqi Sunnis into what he promotes as a vanguard against the Shiite-led government. He offers "repentance" for those who allied with U.S. forces against al-Qaeda in Iraq. Though less an operational commander than Zarqawi, he's just as capable of inspiring followers. In a well-researched profile last December, Time magazine described Baghdadi as "Osama bin Laden's true heir."

A nightmare for U.S. counterterrorism analysts is that these al-Qaeda offshoots could recruit new followers among the millions of desperate Syrian refugees in Jordan, Lebanon, Turkey and Syria itself.

One veteran U.S. official views the terrorist threat coming out of Syria and Iraq as potentially the most worrying development in the Middle East since the late 1970s. America may be less focused on the jihadists than it was a decade ago, but they remain very much interested in the U.S.