Standing just a few strides from the Syrian border, an Iraqi was mingling with Syrian rebel units outside their camp here, trying to find one that would take him in and let him fight in the uprising.

"It's an honor for me," said Sheik Abu Abdullah, wearing the white robe, Islamic skullcap and beard common among Islamic hardliners.

The battle-hungry Iraqi is part of a stream of Arab fighters who have been drawn to the rebel cause, adding not only to the growing complexities of Syria's civil war but also deepening the uncertainties of what could follow Bashar Assad's regime.

After the latest blow to Damascus _ this week's defection of Syria's prime minister _ U.S. Secretary of State Hillary Rodham Clinton said Tuesday that there is an urgent need to plan for what happens afterward if and when the Assad regime falls. She said it is important to ensure that Syrian state institutions remain intact. The hope among U.S. officials is to find a "soft landing" that keeps some degree of stability.

However, few of the imaginable scenarios for post-Assad Syria portend stability after more than 17 months of blood-letting in a country that is more ethnically splintered than Iraq and holds perhaps the greatest international stakes of the Arab Spring.

One scenario: a bloodbath as Syria's majority Sunni population, which has led the uprising against Assad, seeks vengeance against the minority Alawite sect, an offshoot of Shiite Islam that forms the backbone of Assad's regime. The conflict's already increasing sectarian overtones suggest any power vacuum could usher in a direct fight between the two communities.

Another possibility is a free-for-all fight among the "victors" _ the patchwork collection that makes up the anti-Assad revolt but has no common vision for the future. Among them are opposition figures in exile who have some political weight abroad but often haven't set foot in the country in years; political defectors like the prime minister; military generals who broke with the regime; the thousands of low-level soldiers who also defected and are doing much of the fighting in the rebel Free Syrian Army; and Syrian Islamists who have formed militias that nominally fight under the banner of the FSA but do not share the secular vision of some of its other members.

Add to that mix the wild card of the foreign fighters, whose numbers are unknown but who could fuel conflict between Syria's Islamist and secular factions. Some of the foreigners are believed to have links to al-Qaida, while many more are Islamic militants with no direct connection to the terror group.

"The militarization of the uprising provided a cover and a space for everyone _ whether they are fighting to topple Assad, fighting for a free country, fighting a holy war in the name of God or fighting for a state that implements Islamic law," said Randa Kassis, a Paris-based member of the opposition Syrian National Council.

"This doesn't bode well for the future of Syria," she said.

While the foreign fighters share the goal of ousting Assad, they tend to view the fight in terms of a jihad, or holy war, to remove a regime they see as tainted with ties to Shiite Islam and to put in its place a Sunni Islamist rule.

Mohammed Idilbi, a Syrian activist based in Turkey, told The Associated Press there was a significant number of Arabs fighting against Assad from countries including Libya, Yemen, Tunisia and Lebanon. He could not give exact numbers but said his own relatives were involved in bringing Arab fighters into Syria.

Many of them fly to Turkey and _ like Abu Abdullah _ try to link up with one of the units of the Free Syrian Army, which often act with near total autonomy. Abu Abdullah, who fled Iraq several years ago after being jailed and tortured by Shiite militiamen, was so far not finding a unit to take him in, told by some rebels that they needed weapons, not more men. Abu Abdullah's age _ he's older than most fighters _ may also have been a factor. He spoke on condition his full name and personal details not be cited for fear of reprisals.

Other foreign fighters just cross directly into Syria and are met by rebels.

"Now that there are liberated border areas it is easier for the fighters to come," Idilbi said in the Turkish border town of Kilis. "Those Arabs who come say they are coming to fight Iranians and Shiites."

The State Department's counterterrorism coordinator, Daniel Benjamin, said it is believed that the number of fighters directly linked to al-Qaida "is relatively small, but there is a larger group of foreign fighters, many of whom are not directly affiliated with al-Qaida, who are either in or headed to Syria."

"Clearly this is a matter of concern for all who fear greater violence in Syria and for regional stability," Benjamin told reporters. "We are very much alert to this issue. We've spoken with the Syrian opposition groups and warned them against allowing such fighters to infiltrate their organizations. They've assured us that they are being vigilant."

A Free Syrian Army official, Ahmed Kassem, denied there are foreign fighters with the opposition in Syria _ apparently to avoid friction with Western backers. But the rebels also count on even more active aid from Gulf states such as Qatar and Saudi Arabia, which were among the pathways for jihadis joining the Iraq insurgency.

A video released by activists after rebels took over Syria's Bab el-Hawa border crossing point into Turkey last month showed a young man raising an al-Qaida flag and declaring the area a breakaway Islamic state.

A rebel commander identified the man as Egyptian Mohammed Midhat al-Masri, the son of Midhat Musri, who was better known by Abu Khabab al-Masri, a top al-Qaida leader who was killed by the U.S. in 2008. The commander spoke on condition of anonymity because he was not authorized to brief media. His claim could not be independently verified.

Syria's U.N. Ambassador Bashar Ja'afari told the international body in May that government forces killed 12 foreign fighters and detained 26 of different nationalities.

The emerging radical voices also have the potential to change the very nature of the uprising, which began in March 2011 with calls for reform. At the time, protesters and opposition leaders described their movement as entirely secular and free of religious overtones. Analysts warn that the longer the conflict continues, the easier it will be for radical forces to fill the vacuum _ making the violence in Syria far more unpredictable.

The dangers of a failed state in a religiously divided society are all too apparent in Iraq, where the collapse of the region's other Baathist regime unleashed a cycle of vendetta killings and a bloody struggle for power. More than 100,000 American troops were unable to enforce calm.

The diversity of Syrian society _ which includes Sunnis, Christians, Druse, Kurds and Assad's Alawite community _ makes the possibility of a post-Assad power struggle all the more likely. Already, the conflict has seen Alawite gunmen participating in mass killings of Sunni civilians, as well as tit-for-tat slayings of Alawites by Sunnis.

There have been signs that Alawites might try to carve out a breakaway enclave in the region where their community has historically been concentrated along the Mediterranean coast.

Sectarian bloodshed in Syria could also spill over to cause turmoil in neighboring countries. Lebanon and Turkey each have communities with ties to their brethren in Syria, and Lebanon is particularly vulnerable. Even Jordan fears being dragged into the conflict because of the tens of thousands of Syrian refugees on its soil.

Syria is also key to the region's competing alliances. It is closely tied to Iran, while Sunni powerhouses in the Gulf, particularly Saudi Arabia, have sought to break Damascus away from Tehran. Both sides would likely intervene whether covertly or overtly in a post-Assad Syria to try to shape it.

As the chaos mounts, concerns over Syria's long-suspected chemical weapons stockpiles have taken on a new urgency. On July 23, Syria threatened to unleash its chemical and biological weapons if the country faces a foreign attack. It was Syria's first-ever acknowledgment that the country possesses weapons of mass destruction, and the timing of the announcement was seen as a warning against anyone pondering international military intervention in Syria.

Syria is believed to have nerve agents as well as mustard gas, Scud missiles capable of delivering these lethal chemicals and a variety of advanced conventional arms, including anti-tank rockets and late-model portable anti-aircraft missiles.

Israeli leaders have indicated they could attack Syrian arms depots to keep anti-Israel militants from getting their hands on chemical weapons should the Syrian regime collapse.

Despite the dire predictions for the future, rebels say they feel confident. Even though the core of the regime has remained loyal _ including the top echelons of the military _ the pace of defections is increasing, with generals and diplomats joining the opposition.

The regime "is living its last days," a fighter who identified himself as Abu Ahmad told the AP by telephone from the northeastern Syrian town of Jarablous. "Every time our youth hear that an officer or an official defected, it boosts their morale."

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Kennedy reported from Beirut.