AMMAN (Reuters) - As President Bashar al-Assad has concentrated his forces on wresting back control of Syria's business hub Aleppo, rebels have slowly gained ground in the eastern tribal heartland where the big prize is the country's oil wealth.
From fortified outposts in the crude-producing desert near Iraq, government forces have been shelling Deir al-Zor, an impoverished Sunni Muslim city on the banks of the Euphrates that anchors a vast, arid region bordering Iraq.
But core loyalist troops, drawn mostly from Assad's minority Alawite sect, are locked primarily into what is shaping up to be a protracted battle for Aleppo, as well as in shoring up an uncertain grip over the capital Damascus.
In the process, Assad faces the specter of Deir al-Zor province slipping out of his orbit and with it Syria's 200,000 barrel-a-day oil output, military experts and diplomats say.
Residents said that over the past three months rebels had extended control over wide tracts of Deir al-Zor as over-stretched loyalist forces have retreated to security compounds in the centre of the provincial capital and its outskirts.
Rebels say they control at least half of Deir al-Zor city, and oil production has been almost halved since the 17-month-old uprising began as Western sanctions have deprived Damascus of its main clients for Syrian crude in Europe.
A Western diplomat monitoring the Syrian military said that rebel forces in Deir al-Zor were fragmented but that Assad's forces lacked the numbers and supply lines to defeat them.
"There are lots of security forces in Deir al-Zor and these are more vulnerable to armed assaults by rebels. With the core army forces in Damascus, one has to ask when will regular troops in areas such as Deir al-Zor just quit," the diplomat said.
"With the use of artillery, which is a blanket weapon, against (populated) neighborhoods, the regime propaganda about the army fighting terrorists must be wearing thin."
Muhaimen al-Rumaid, a coordinator for the Syrian Rebel Front, said army reinforcements to Aleppo were being sent from Hasaka and Raqqa, provinces adjacent to Deir al-Zor, exposing the military to attacks from behind.
The picture in Deir al-Zor city, with a population of 600,000, and its thinly populated hinterland is complicated by a web of regional and foreign links Assad and his late father forged but which are now in flux, opposition sources said.
The army and loyalist militia known as shabbiha still draw a significant portion of its Sunni rank and file from poorly educated Sunnis in rural regions of Deir al-Zor.
Fighters with sympathies for al Qaeda have come in from nearby Iraq and carried out three suicide attacks against Assad's forces in Deir al-Zor, rebel sources said.
Assad has focused his vastly superior firepower on wiping out rebels in certain major cities, sometimes wreaking widespread destruction, only to find insurgents taking advantage elsewhere. Now there is fighting in most provinces.
Andrew Terrill, Middle East expert at the U.S. Army War College, said Assad's "siege warfare" to keep rebels in Deir al-Zor in check has developed into a war of attrition to his disadvantage, with insurgents gradually acquiring weapons capable of hitting warplanes and disabling tanks and artillery.
"Assad is not going to be dealt a fast defeat in Deir al-Zor, but his military is getting eroded. If Deir al-Zor falls I am not sure he can commit crack troops or manage a re-supply to get it back," he said.
In a morale boost for the armed opposition, rebels said on Monday they had captured the pilot of a fighter jet after shooting down his aircraft in the area of Mou Hassan, about 20 km (12 miles) east of the city of Deir al-Zor. State media said the plane crashed due to technical problems.
Every day Deir al-Zor city comes under fire from the rockets of helicopter gunships as well as artillery and tanks deployed on its fringes and deeper into the surrounding desert, killing and wounding dozens of civilians, according to residents.
"Whenever the army approaches to take one neighborhood the rebels repel it so the regime seems content with relying on indiscriminate shelling," said Abu al-Tayyeb al-Deiri, an opposition activist who has been keeping tabs on casualties.
Most government departments have shut and public workers are unpaid in what activists call collective punishment of a tightly knit population siding increasingly with rebels after alliances between the Damascus elite and tribal chiefs unraveled.
"The regime is trying to undermine popular support for the (rebel) Free Syrian Army. It has cut basic services and for most of the past month we have been without water or electricity or phones," Sameh, a school teacher, who gave only his first name for fear of retribution, told Reuters by telephone.
"Luckily we have a big tank for irrigation and we have been using that as drinking water...(But) I have not been paid for months...The people are generally with the rebels," Sameh said.
He cited regional discontent with Alawite rule that spread in the past few years as unqualified Alawite teachers and other public workers came in from the Mediterranean coast on the other side of the country to be handed managerial jobs while local Sunnis languished in lower posts.
Another incubator of grassroots support for rebellion has been a water shortage that developed over the past six years. It left thousands in the province dependent on foreign food aid as Damascus kept withholding infrastructural investment because the region was not seen as important to the central power structure.
Deir al-Zor ("monastery of the grove") fell into decay years ago with unpaved streets and shabby, uniform Soviet-style buildings contrasting with a suspended foot bridge built over the Euphrates by French colonial administrators in the 1930s.
The defection of Prime Minister Riyad Hijab and Syria's ambassador to Iraq, both tribal figures from Deir al-Zor, has given the revolt a further morale boost, residents said, but done little to change military facts on the ground as aerial and ground bombardment have hindered decisive rebel advances.
An estimated one-third of Deir al-Zor city's inhabitants have fled to the bordering provinces of Hasaka and Raqqa, according to witness and opposition accounts.
Fadi Abed, a merchant from Damascus who was stuck in Deir al-Zor for weeks before fleeing with six families to the Christian Valley region near the border with Lebanon, said basic supplies such as water and powdered milk were mostly unavailable and that most bakeries could not operate.
"A (government) roadblock cut off the northeastern entrance of Deir al-Zor and the army has confiscated medicine and food aid that was being sent to the city from Hasaka," Abed said.
Additional reporting by Oliver Holmes; Editing by Mark Heinrich