The U.S. and its Middle East allies are bracing for the potential that a catastrophic fracture of Syria along sectarian lines could spread chaos into neighboring countries.

Such a breakdown inside Syria is now seen as a likely outcome if rebel fighters succeed in quickly overthrowing President Bashar al- Assad´s regime, American, Israeli and Arab officials said.

Under such a scenario, Mr. Assad´s ethnic Alawite clan, an offshoot of Shiite Islam, is expected to retreat to its stronghold in the country´s western coastal regions, where it would consolidate its forces and battle its foes.

That would leave moderate and fundamentalist Sunni groups, Kurds and Christians—without their own enclaves to which they can retreat—to compete to fill the power vacuum in other parts of the country.

"We are already heading to a partition of the country," said a U.S. official who is closely tracking Syria. "There are a lot of parts of the country the regime can´t hold."

A rebel attack on Wednesday killed four top Assad aides in a surprising display that officials said could prove a turning point— though in which direction is uncertain. "This could tip fast and decisively away from the regime, or the regime could harden up and brutalize their way back to equilibrium," a U.S. diplomat said.

U.S. and Mideast strategists fear that a rapid collapse would have far-reaching effects on Syria´s neighbors, particularly Iraq, Jordan, Israel and Lebanon.

"Syria has become a convenient battlefield for everyone, a place to divide the Arab world, said Farid Khazan, a Lebanese lawmaker and a professor of political science at American University of Beirut. "You won´t be able to reshape that country without messing up the entire region."

Tensions between Sunnis and Shiites in Syria have already destabilized communities in northern Lebanon and Iraq, according to U.S. and Mideast strategists. In Jordan, officials fear that the rise of fundamentalist Sunni groups in Syria, particularly the Muslim Brotherhood, could threaten King Abdullah´s monarchy.

The gathering chaos in Syria, which is believed to have stockpiles of chemical weapons, is increasing pressure on the Obama administration to play a more direct role in trying to shape a post-Assad government.

In recent weeks, U.S. and Arab officials and Syrian opposition groups said they have seen an increasing dissolution of the Assad regime´s control over the country. This dynamic is most pronounced in tribal regions in the south and areas near the Turkish border.

The Free Syrian Army, which oversees the country´s myriad rebel groups, has become so confident that it is considering declaring Syria´s Idlib province a liberated zone, according to members of the group.

The FSA and Syrian opposition groups are providing social services and governance to local populations, opposition officials and analysts say. Revolutionary councils in rebel-held areas have begun supplying cooking gas and other supplies.

U.S. and Arab officials, however, said they have seen growing signs of the Assad regime trying to carve out an ethnic enclave inside Syria.

Recent regime campaigns in cities such as Hama and Homs have focused on driving out Sunni communities, the officials said. Some of the Syrian Alawite leaders have begun to move family members back to the coastal city of Latakia and to secured mountain areas in the western region, they said.

"The Alawites would essentially fall back and create a ministate, giving up Damascus," a U.S. official said. "The ethnic cleansing, the sectarian cleansing of that area would be consistent with that."

The conflict has huge ramifications for neighboring countries. As violence rises, Syrians are fleeing in ever-larger numbers. Tens of thousands of Syrians have fled to Lebanon since Thursday, while thousands more are pouring into Iraq by land and by air, and Jordan says that more than 100,000 Syrians are now within its borders.

Lebanon has already had several sectarian clashes between supporters and opponents of Syria´s regime. Lebanon also is a stage for regional rivalries to play out between Saudi Arabia and Iran, which back feuding Lebanese political factions.

"I don´t think Lebanon has ever been through a situation this sensitive and complicated as right now. The divisions are very deep," a senior Hezbollah official said.

In Iraq, the Syrian chaos could easily translate into radical Sunni jihadists crossing the border with money and weapons to target the Shiite-dominated government with car bombs and assassinations.

Security officials in Israel fear their northern neighbor could become a haven for terrorists, as have other broken states. That could transform Israel´s long, quiet border with Syria into another battleground. The potential that rogue actors could launch rockets at Israel from Syrian territory, as Hezbollah and Hamas have done in the past, is a deep concern.

The U.S. and its allies also worry Syria´s stockpiles of chemical weapons could wind up in the hands of Islamist militants. U.S. officials have acknowledged contingency plans to send American forces to secure weapons sites if necessary, but say they would prefer to see allies, chiefly Jordan and Turkey, take the lead.

However, the likelihood of an orderly political transition in Syria, in which international peacekeepers could provide for calm, appears less and less realistic. "The fall of the Assad regime doesn´t mean it is the end," said Jihad Zein, editorial writer for Lebanon´s largest daily newspaper, An Nahar. "We will have a chaotic Syria and some kind of Islamist party dominating the street for a long time."

—Charles Levinson and Joshua Mitnick contributed to this article. (Copyright © Dow Jones & Company, Inc.) 07/21/12)