The Islamic State group may have suffered several defeats along the periphery of its so-called caliphate, but so far no one has been able to challenge its control over the core of its territory. While the US-led coalition has escalated its airstrikes on the Islamic State's oil infrastructure, the allied nations have shown little appetite for the grueling ground fight that would be needed to actually drive the group out of Syria's east. And although Raqqa seems to serve as the Islamic State, or ISIS', symbolic and administrative center in Syria, it is Deir al-Zour province that provides much of its oil revenue and serves as a critical link between its Syrian and Iraqi territories.

That may be about to change, thanks to a new force of eastern Syrian rebels with a singular focus on the Islamic State and, local sources say, quiet backing from the United States.

Known as the New Syrian Army or NSA, it first appeared in November in a YouTube video under its Arabic name, Jaish Suriya al-Jadid. Its aim is to retake Deir al-Zour, and it seems to have copious American weaponry and air support from the coalition on its side.

Based on interviews with involved rebels and informed local activists reached via Skype and social media, it is clear the NSA faces tough odds. Its numbers are reportedly few, in part because some Deir al-Zour rebels distrust its American backers. Yet the group, drawing strength from deep-seated local enmity towards the Islamic State, might still offer the best hope of pushing ISIS out of a key province.

When fighting between ISIS and a broad cross-section of Syria's rebels broke out across the country in January 2014, ISIS initially contracted into a few eastern Syrian strongholds before rebounding west. Deir al-Zour, however, resisted stubbornly. Local Free Syrian Army or FSA units and the eastern branch of Syria's al-Qaeda affiliate, Jabhat al-Nusrah, managed to fend off ISIS until the summer of last year. But then the stunning June 2014 ISIS capture of Mosul in neighboring Iraq effectively meant the end for the resistance in Deir al-Zour. Islamic State fighters took their newly captured American weaponry and turned west, encircling the city. The last of the local rebels fought a desperate battle before surrendering or fleeing west across Syria's central desert.

Since then, ISIS has made Deir al-Zour pay for its courage, and for its continuing resistance, which has included assassinations of ISIS members. In the most egregious example, ISIS retaliated against the local Sha'eitat tribe, which had staged a rebellion in the summer of 2014, by killing more than 700 of its members – and teaching the people of Deir al-Zour a terrible lesson in the process.

"By ISIS's own admission, Deir is the area that most resisted. That cost them men and materiel," said one Deir al-Zour activist who was interviewed via social media and agreed to speak on the condition of anonymity.

Deir al-Zour sits at the dark center of ISIS's Syrian and Iraqi territory, and news from the province is comparatively sparse; it has some activist news outlets, but has received only a fraction of the coverage Raqqa has. Yet ISIS has been no less brutal in Deir al-Zour than in Raqqa.

"Al-Raqqa is totally normal compared to what's happening in Deir al-Zour," said another activist. And the bad blood is mutual. "There's an old grudge between people [in Deir al-Zour] and ISIS," said activist Omar Abu Layla. "If the opportunity comes, they'll fight, even if they have to do it with knives."

But so far there haven't been many willing or able to take up arms against what they see as an occupying force, and Deir al-Zour's seasoned rebels have been scattered across western Syria. Jabhat al-Nusrah's eastern branch has seen its numbers dwindle since fleeing Deir al-Zour for southern Dara'a province, where it has mostly occupied itself fighting a war of attrition with a local ISIS affiliate. Deir al-Zour's FSA rebels have settled mostly in the Qalamoun mountains north of Damascus and in Syria's central Badiyyah desert, where they have clashed with ISIS and helped frustrate its attempts to advance south.

Enter the New Syrian Army, which has largely recruited from this Deir al-Zour rebel diaspora. The NSA has given its fighters what it says is professional training, as well as American-made weaponry – its promotional video shows fighters using weapons including the Mk 14 Enhanced Battle Rifle, M240 machine gun, and M120 mortars. In the video, field commander Khaz'al al-Sarhan vows the Army stands "responsible before God and Syrian people for restoring security to this pure country and expelling Da'esh [ISIS] and those who support it from the whole of Syria." The NSA was initially trained in Jordan, sources close to it said, but is now based inside Syria. It is part of the existing Jabhat al-Asalah wal-Tanmiyyah (Authenticity and Development Front) rebel coalition, which has a moderate Salafist platform -- Islamist, but vehemently opposed to ISIS and jihadist excesses -- and fields units across the country.

On November 15, the New Syrian Army launched its first operation, an attack on ammunition storehouses and a bomb factory at the al-Tanf border crossing with Iraq. In a subsequent promotional video on the raid, the NSA's commander Col. Muhannad al-Talla' hailed the attack as "the first step towards liberating the east." He also said it had been successful in part because of the "close cooperation" between the New Syrian Army and the international coalition.

"There's coordination in a number of forms, including logistical support and support for organization and missions, even air strikes -- without harming civilians," said a New Syrian Army spokesman who goes by the alias Abu Yousef.

The group's arrival on the scene has not been without controversy. Some Syrian activists and commentators have complained that the Army's introductory video made no mention of the Assad regime, instead focusing just on ISIS. That troubles some potential allies, who want to rid Syria of its dictator first.

"The fundamental target is the regime, because its fall will mean the end of all this chaos," Khaled al-Hammad, secretary-general and head of Jabhat al-Asalah wal-Tanmiyyah, told VICE News. "But Da'esh is the one in control in the east, so it's the prime target there." Asked if the NSA would fight if it came face to face with regime forces, al-Hammad said, "Definitely."

Deir al-Zour sources say that local rebels are also divided over the role of the NSA's external backers, in particular the US. General distrust of the Americans, these sources say, has stopped other Deir al-Zour rebels from joining the Army. These rebels doubt the United States' desire to bring down the Syrian regime, and many are unhappy with the civilian toll of the coalition bombing campaign in eastern Syria. (Approached for comment on the NSA, a CIA spokesperson referred VICE News to the Department of Defense. A U.S. Department of Defense spokesperson said she had no information beyond what has been reported in the media.)

Deir al-Zour's rebels have been stung before; many are still bitter that rebels in northern Syria left them to fight ISIS alone, and they fear that any support to fight the Islamic State from an external backer will suddenly fall away.

In the meantime, the NSA's manpower is an obvious problem – its official force strength is in the hundreds, but some interviewees gave much lower estimates. "Lots of young men won't fight again until they can see the credibility of these commanders and those providing support," said one activist. "Then the numbers will go up."

Still, there are reasons to think the Army might be more successful than other recipients of visible U.S. support,.Its members and leaders are longstanding, respected Deir rebels. Jabhat al-Asalah wal-Tanmiyyah has contributed to the fight against the Assad regime across the country, but it is especially popular in the east, where its factions initially helped liberate the province from the regime.

In any case, there may be no one else willing or able to take back Deir al-Zour. "You can't push a western brigade, or even the Kurds, to take Deir al-Zour. Only the people of Deir al-Zour will take it," said Abu Layla, the local activist. The limitations of a Kurdish-led offensive on ISIS from the north are increasingly apparent; the northern Kurdish and Arab militia in the "Syrian Democratic Forces" have made clear they have their own interests that do not include fighting to liberate Deir al-Zour. The Syrian regime, meanwhile, still holds part of Deir al-Zour's provincial capital and a military airbase near the city, but is more interested in defending its western strongholds than fighting to retake the desert east.

So it will fall to Deir al-Zour's rebels. After all, they are the ones who "want to liberate their homes and people from this monstrous organization," said al-Hammad.

Abu Layla put it in stark terms: "If the sons of the east don't seize this opportunity, there were never be another one," he said. "You can forget Deir al-Zour forever."

_Follow Sam Heller on Twitter: _@AbuJamajem