BEIRUT — After the battlefield of Iraq's Mosul, the next major campaign against the Islamic State group will be to take its de facto capital, the Syrian city of Raqqa. The Pentagon has drawn up a secret plan to do that, likely leaning on local allies with stepped up American support.

The question is: In the tangled mess that is Syria's conflict, who are those local allies?

Syrian government forces, Turkish troops and their Syrian militia allies, and U.S.-backed Kurdish forces all have their eye on Raqqa. Each vehemently rejects letting the others capture the city and would likely react in anger should the United States support the others. And it is not clear that any has the resources to take the city on its own.

"Raqqa is more of an abstract goal: everyone wants it in principle, but no one is willing to commit the resources and bear the risks necessary," said Faysal Itani, an analyst at the Washington-based Atlantic Council.

The fall of Raqqa, the Islamic State group's de facto capital and largest remaining stronghold, would be the biggest defeat for the militants in Syria since they captured the northern city on the banks of the Euphrates River in January 2014.

President Donald Trump has vowed to "obliterate" the group. "We will work with our allies, including our friends and allies in the Muslim world, to extinguish this vile enemy from our planet," he told Congress on Tuesday.

The top U.S. commander in the campaign against IS, Lt. Gen. Stephen Townsend, has said he believes Raqqa and Mosul will be taken within six months. So far, the offensive on Mosul has been underway four months, with only half the city captured from the militants in ferocious street-to-street urban combat. And that is using a relatively intensively trained and united military, backed by heavy U.S. firepower and commandos on the ground — a contrast to the comparatively undisciplined and fragmented forces the U.S. has to choose from as allies in Syria.

Raqqa is a smaller city than Mosul, but the militants are believed to have dug in with powerful fortifications there.

In Syria, U.S-backed predominantly Kurdish fighters known as the Syria Democratic Forces, or SDF, remain Trump's best bet. Aided by U.S.-led coalition airstrikes and some 500 U.S. special forces troops deployed in an advisory role, the force has been marching toward Raqqa since November. Closing in on the city from different directions, it is now stationed some eight kilometers (five miles) north of the city.

The U.S. military recently provided a small number of armored vehicles to the U.S.-backed force to give better protection from small arms fire and roadside bombs as they get closer to Raqqa.

Further aid to the rag-tag group, however, raises sensitive questions over how to deal with Turkey, a NATO ally with much at stake in Syria. Turkey considers the main Kurdish militia in Syria — known as the YPG, and an affiliate of the U.S.-backed SDF — a terrorist organization, and has vowed to work with Syrian opposition fighters known as the Free Syrian Army to liberate Raqqa.

In a dramatic reversal of years of the Obama administration's calls for the ouster of President Bashar Assad, Trump has hinted he might be willing to work with Assad's army and Russia, whose year-and-a-half military intervention has propped up Assad's government.

Assad's forces are preoccupied with other battles, however, and would likely need significant U.S. military involvement to take on Raqqa. On Wednesday, the Syrian military recaptured the central town of Palmyra, a city located in the desert south of Raqqa that has gone back and forth between control of the military and the extremists several times. The government forces have also clashed with the Turkish-backed Syrian fighters, who block their path to Raqqa.

Syrians are sharply divided over who should enter Raqqa. Many opposition supporters consider the SDF, which maintains a tacit non-aggression pact with Assad's forces, to be a hostile group. There are also fears of tensions if Raqqa, home to a nearly 200,000 mainly Arab population, is taken by the SDF, a coalition of Kurdish, Arab and Christian fighters.

"Let us be frank that any force that will liberate Raqqa, other than the Free Syrian Army, is going to be a new occupation force with different flags and banners," said Mohammed Khodor of Sound and Picture Organization, which tracks atrocities by IS in Iraq and Syria.

Turkish Prime Minister Binali Yildirim was even more blunt, warning that if the SDF enters Raqqa, it will hurt relations between Ankara and Washington.

"We have said that a terror organization cannot be used against another terror organization," the Turkish leader told the state-run Anadolu news agency.

The Kurds reject that notion and insist that only forces fighting under the SDF banner will liberate Raqqa.

"Turkey is an occupation force and has no legitimate right to enter Raqqa," said SDF spokeswoman Cihan Sheikh Ehmed. In a text message exchange from northern Syria, she said the SDF has the experience in fighting IS to finish the operation.

Battlefield victories by the SDF against the Islamic State group have brought growing Western support. Asked if adding more U.S. troops or better arming Syria's Kurds were options, Defense Secretary Jim Mattis said he will "accommodate any request" from his field commanders.

In Mosul, the U.S.-led coalition is playing a greater role than ever before in the fight against IS and coalition forces have moved closer to front-line fighting.

U.S. Air Force Col. John Dorrian says the increased support is an effort to "accelerate the campaign" against the Islamic State group, noting that launching simultaneous operations in both Mosul and Raqqa "puts further strain on the enemy's command and control."

"It is a complicating factor when you don't have a partner government to work with," conceded Dorrian, adding that whoever the coalition partners with in the fight for Raqqa is "a subject of ongoing discussions."

Wladimir van Wilgenburg, a Middle East analyst at the Jamestown Foundation who closely follows Kurdish affairs, says the U.S.-led coalition wants to have a quick end to IS in Raqqa, from which external operations against the West are planned. That means it would prefer to work with the Kurdish-led SDF forces "since they are able to mobilize manpower unlike the Turks," he said.

In any case, the battle for Raqqa is sure to be a long and deadly one. It took the SDF nearly 10 weeks to capture the northern Syrian town of Manbij from IS last year. It took Turkish forces and allied groups more than three months to retake the town of al-Bab, a costly battle that killed dozens of Turkish soldiers and many civilians.

Raqqa is much larger than either Manbij or al-Bab. Some Syrian opposition activists say the extremists dug a trench around it to make it difficult for attackers to storm it.

"It would be difficult for any troops," said Itani of the Atlantic Council.

"Witness the slow and ugly progress in Mosul as well. Raqqa would be tough," he said.

Associated Press writer Susannah George in Mosul, Iraq, contributed to this report.