In expanding its airstrikes into Syria against Islamic State extremists, the U.S. could find itself entangled in a morass of jihadis, rebel rivalries and religious hatred.

Unlike Iraq, the U.S. has no firm allies inside Syria to take over areas if fighters from the Islamic State group are pushed back. Unless the West decisively backs the outgunned moderate rebels, it risks the unintended consequence of prolonging the widely discredited rule of President Bashar Assad.

President Barack Obama said Wednesday that in the fight against the Islamic State group, the U.S. "cannot rely on an Assad regime that terrorizes its own people — a regime that will never regain the legitimacy it has lost."

"Instead, we must strengthen the opposition as the best counterweight to extremists like ISIL, while pursuing the political solution necessary to solve Syria's crisis once and for all," Obama said, referring to the Islamic State group by one of its acronyms.

But it's a lot more complicated.

Northern and eastern Syria, where the Islamic State group controls territory and where the U.S. is likely to strike, is a landscape shattered by three years of war and rife with conflicting loyalties and rivalries.

Assad is supported by members of his Alawite sect, an offshoot of Shiite Islam, as well as some Christians and other minorities. The Islamic State group and other rebel factions are overwhelmingly Sunni Muslims, Syria's majorly sect.

The Islamic State group controls territory stretching from the outskirts of the northern city of Aleppo to the eastern border with Iraq — roughly one-third of Syria.

Initial U.S. airstrikes are likely to focus on Raqqa, the self-declared capital of the group's proto-state along the Euphrates River, but the group has strongholds in northern Syria, including the towns of Manbej and al-Bab. Only few kilometers (miles) separate these towns from front lines occupied by rival rebels. Assad's forces, beaten back from all their bases in Raqqa province, are still holed up in a major air base in the eastern Deir el-Zour province.

The group might also be hiding its assets and fighters among the civilian population, making it harder for the U.S. to carry out the attacks.

Activists reported Thursday that the group was evacuating and pulling out some of its hardware from al-Bab and Raqqa, away from urban centers. The claims could not be independently confirmed.

"A tricky situation is going to be how the U.S. will ensure that it is only ISIS targets that are hit, rather than civilians and others," said Haras Rafiq of the Quilliam Foundation, a London-based think tank.

The U.S. would most likely target supply lines and heavy weapons to erode the group's power, rather than wiping it out, said Eliot Higgins, who analyzes weapons used in Syria. "It's really about targeting the stuff they can't hide easily."

Higgins said the U.S. was unlikely to attack militants in cities and towns, where airstrikes could cause civilian deaths and give the group a propaganda victory.

It also would also be difficult to target them if they were attacking government assets, such as military bases, because the U.S. would "end up being air support for the Syrian government," he said.

That leaves supply lines and heavy weapons, such as machine-gun mounted armored vehicles, which the militants have used to devastating effect in Iraq and Syria.

Another question is whether Assad would retaliate against U.S. airstrikes.

Syria's air defenses have taken a major hit in the country's north and east, but the air force is still active, firing rockets and dropping crude bombs on rebel-held areas on daily basis.

Assad could try to drag in his backers — Iran and Russia — getting them more involved diplomatically or otherwise.

Syrian opposition activists and rebels said U.S. airstrikes would do little to push back the Islamic State without robust support for the armed opposition to Assad.

"There's no debate that the Americans can hit from the air, but the big question is, will they do it without the opposition to take the areas after they have been hit?" said Hassoun Abu Faisal, an Aleppo-based activist.

Aron Lund, a Swedish researcher on Syrian groups, said it remained to be seen whether U.S. airstrikes would end up helping the Assad government or the rebels.

"If the U.S. manages to roll back the Islamic State from areas like in Aleppo or Hassakeh, then that vacuum needs to be filled by somebody," Lund said. "The Islamic State captured those areas by default because they were the strongest group to govern them. That's part of the equation that needs to be changed. Can (the U.S.) train enough fighters to do that?"

As in previous efforts, the U.S. will struggle to pinpoint groups it can arm and train, because of the lack of truly secular factions. The struggle is made more difficult because most of the groups that have successfully waged war against the Islamic State group are far too extreme to obtain Western support.

The U.S. has armed and trained rebel groups like the Hazm Movement in northern Syria, and a coalition called the "Southern Front." But neither has much experience fighting the Islamic State group, said Aymenn al-Tamimi, an expert on Syrian and Iraqi militants.

The battle-hardened secular Syrian Kurdish group known as the YPG has also successfully battled the Islamic State group, most recently entering Iraq to fight them there. But the group is effectively a branch of a Kurdish-Turkish group that's designated a terrorist organization by NATO.

"The U.S. criteria exclude the vast majority of rebel groups in Syria," said al-Tamimi.

The Nusra Front and Ahrar al-Sham both pushed back the militant group in fighting that began in January, killing thousands of rebels.

Ahrar al-Sham lost a dozen of its leaders, including the group's chief, in a bombing Tuesday night, further throwing into doubt its fighting abilities, at least in the short term. The fact that no one knows whether it was Assad's forces or the Islamic State group that was behind the bombing underscores how muddled the situation is.

Rafiq, the analyst, said another unintended consequence of U.S. airstrikes could be the empowerment of other extremist factions like the Nusra Front, as fighters rejoin it after defecting to the Islamic State group.

In the meantime, the door is open for Assad to carry out even further strikes on his enemies.

"It gives him the license to actually become a lot more aggressive," Rafiq said.

Rebel activist Abu Yazan, based in eastern Damascus province, said supporting the armed opposition was crucial for winning the hearts and minds of Syrians living in areas controlled by the Islamic State group.

"If you only fight the Islamic State, and the regime drops barrel (bombs) on those areas, it's hard to convince people to support (the U.S. airstrikes) because they will see it as supporting Assad," he said.