Baghdad, Iraq ? The killing of Abu Musab al-Zarqawi raised hopes that Iraq’s slide toward civil war or sectarian disintegration could be arrested, but there are signs that Shiite-Sunni antagonism may now be too deeply rooted.

Al-Zarqawi, a Jordanian-born Sunni Muslim, played a key role in stoking Shiite-Sunni tensions the past three years, ordering hundreds of bloody attacks against Iraqi Shiites and issuing vitriolic tirades seeking to deepen a religious schism that dates back 1,400 years.

On Sunday, his feared terror group, al-Qaida in Iraq, said there would be no letup in its combat operations. It vowed in an Internet statement “to prepare major attacks that will shake the enemy like an earthquake and rattle them out of sleep.”

Gen. George Casey, commander of U.S. forces in Iraq, said he took the threat seriously, but suggested a surge in al-Qaida rhetoric also was evidence that the insurgency is “hurting.”

“Now, that said … it still has the capability to generate terrorist attacks across Iraq,” he said on CBS’s “Face the Nation.”

It is not just al-Qaida playing on the religious divide.

On Saturday, the Mujahedeen Shura Council – five allied groups in the Sunni Arab-dominated insurgency – sent an Internet condolence message for al-Zarqawi’s death that echoed the former al-Qaida chieftain’s hatred for Shiites.

“As for you the slaves of the cross (U.S.-led coalition forces), the grandsons of Ibn al-Alqami (Shiites), and every infidel of the Sunnis, we can’t wait to sever your necks with our swords,” council leader Abdullah bin Rashid al-Baghdadi wrote.

The specter of civil war or a breakup of Iraq has grown steadily since shortly after the U.S.-led invasion in 2003 ousted Saddam Hussein, which allowed the long-oppressed Shiite majority to assume dominance over minority Sunni Arabs who had run the country for decades.

Sunnis, embittered over their loss of power and status, formed a variety of insurgent groups, although it was al-Zarqawi who grabbed international prominence in the fight to drive U.S.-led troops out of Iraq.

In time, his organization added Shiite Muslim civilians to its target list, killing thousands and, on Feb. 22, desecrating a major Shiite shrine. The latter set off a storm of increasing sectarian violence that has shown no sign of slowing since al-Zarqawi was killed Wednesday by two 500-pound bombs dropped by U.S. warplanes.

Baghdad, a mixed city of 6 million people that once enjoyed a reputation for tolerance, has become a sectarian battleground. It slowly is becoming a divided city, with the Tigris River marking the front line between Shiites and Sunnis.