As U.S.-backed forces pursue high-profile offensives on Islamic State's two remaining major strongholds, the militants are quietly bracing to make a last stand in a remote resource-rich Syrian province, according to residents and the group's fighters.
Military commanders and families of Islamic State fighters have been bused to Deir Ezzour province, they said, from both Mosul, in Iraq, and Raqqa, the de facto capital in Syria of the group's self-proclaimed caliphate. Both cities are under intense pressure from the U.S.-led coalition.
The militants are regrouping in Deir Ezzour because of its economic and strategic importance as a hub of oil and agriculture, according to Western officials and current and former Islamic State fighters. With the group's survival at stake, its financial needs outweigh the symbolism of holding Raqqa.
"The red line is where the oil and the resources exist, it will be protected as much as possible. It's not important where Islamic State is located--we proved that we can come back anywhere, anytime," an Islamic State commander said in a Skype interview. "But places like Deir Ezzour are irreplaceable."
The influx in recent weeks has evicted locals from their homes. Islamic State explains the ousters by pointing to old and often distant ties local families have to the Syrian regime or the Free Syrian Army, an opposition rebel group--ties the extremists ignored when they were on the rise, residents said.
The move to Deir Ezzour reflects the group's evolution, according to the commander. Islamic State can withdraw from cities yet still torment residents with suicide bombings and other disruptive attacks. As the group loses its ability to conquer and administer territory, it is turning to hit-and-run terror tactics.
With Islamic State running out of havens, Deir Ezzour is the "natural fallback" option, said Col. John Dorrian, Baghdad-based spokesman for the U.S.-led coalition.
Deir Ezzour province is on the border of Syria and Iraq, and it contains the only remaining land bridge where Islamic State leaders and foot-soldiers can move between the two countries.
The route has remained accessible even as the coalition steps up efforts to oust Islamic State from major population centers and sever its supply lines. Iraqi forces launched an offensive to retake Mosul, Iraq's second-largest city, in mid-October. The campaign to recapture Raqqa began weeks later.
"My concern is that we're not really thinking about Deir Ezzour, it's on no one's radar," said a Western diplomat based in the Middle East whose country is part of the U.S.-led coalition. "It's a 'later' problem for the coalition."
Col. Dorrian said the coalition is keeping an eye on the militants' movements in the province.
Islamic State's main points of control in the province are the small border city of Boukamal and nearby al-Mayadin, on the Euphrates River near the al-Ward, al-Tanak and al-Omar oil fields, which hold Syria's most significant reserves.
Islamic State generated as much as $500 million in revenue from oil sales in 2015, a senior U.S. administration official said earlier this year, with much of that oil coming from Syria.
Although the U.S.-led coalition has targeted Islamic State's oil infrastructure--and limited access to the Turkish border, where many smugglers are based--the militants continue to sell oil to the Damascus government and rival rebel groups. Islamic State has lost control of oil fields in Iraq in recent coalition offensives, giving its oil holdings in Deir Ezzour even greater importance.
"In Iraq, they may have enough [oil] to steal and refine for their own use, but not make millions of dollars off of it like they use to," said Col. Dorrian. "But they still have the ability to make a profit to some extent in Syria."
The money they make is vital to funding the group's affiliates abroad, according to jihadist comments in online chat rooms. Western officials said the militants likely use hawalas, or informal money transfers, and cash couriers to spread the wealth. Islamic State's ability to launch international attacks is essential to its image as a terror group with global reach, and the more money it can send to affiliates before it loses control of oil fields, the longer that image will endure, the officials said.
Deir Ezzour is also significant because Islamic State has used it to store crude chemical weapons it has manufactured, according to recent midlevel defectors. The militants lack the sophistication to use the weapons to harm well-protected U.S. forces on the battlefield, Western officials said, but the group has used chemical weapons effectively against civilians and often poorly-equipped local forces the U.S. relies on to combat Islamic State.
Taking Deir Ezzour could prove difficult. In June, Islamic State surrounded and slaughtered members of a U.S.-backed local force called the New Syrian Army that was attempting to retake Boukamal and divide the caliphate in two.
The province's population has been isolated since 2014, when Islamic State captured much of its territory in Syria and Iraq, and there are few local forces with which the U.S. could form partnerships to launch an offensive against the militants.
The Syrian government and its Russian allies, moreover, maintain a toehold in the provincial capital, also known as Deir Ezzour. Their presence complicates the U.S.-led coalition's ability to use airstrikes against the militants.
In September, coalition warplanes mistakenly struck Russian-backed Syrian troops in the province, which helped lead to the collapse of a cease-fire Washington and Moscow had brokered between rebels and the regime.
Moscow might look for ways to disrupt future coalition airstrikes on Deir Ezzour, according to Jennifer Cafarella, a Syria analyst at the Institute for the Study of War, based in Washington. Russia said in October it was sending advanced antiaircraft systems to Syria.
Those systems could be sent to Deir Ezzour, she said, where they could be "part of the bargaining chip Russia continues to play with the U.S. for leverage in Syria."