U.S.-backed Syrian Democratic Forces, made up of an alliance of Arab and Kurdish fighters, advance in the village of Sabah al-Khayr on the northern outskirts of Deir al-Zour as they encircle the Islamic State bastion of Raqqa in February. (Delil Souleiman/AFP/Getty Images)

 U.S.- and Iran-backed forces are locked in a race to take Islamic State strongholds in southeastern Syria and seize a stretch of land that will either cement Tehran’s regional ambitions or stifle them. 

The scramble for pole position in Deir al-Zour province is likely to be one of the most consequential fights against the extremist group in Syria, posing a regional test for President Trump as his administration turns up the rhetoric against Iran.

On Wednesday, the Islamic State asserted responsibility for twin attacks in Tehran that left at least a dozen people dead, a clear reminder of the group’s reach as it faces off against Iranian and U.S. forces and the proxies they support in Syria and Iraq. If confirmed, they would be the group’s first major strikes in Iran.

U.S.-backed forces launched an offensive this week to push the Islamic State out of its self-proclaimed capital of Raqqa. But there are signs that the battle that follows, in the eastern province of Deir al-Zour, will be tougher still, and have greater consequences for the group’s long-term survival as a force holding significant territory.

Experts say the Islamic State has moved senior leaders into Deir al-Zour, along with a growing number of foot soldiers as it loses control of Mosul, the group’s “capital” in Iraq, and internationally backed forces move in on Raqqa.

Located between Raqqa and the Iraqi border, the city of Deir al-Zour is the largest urban center in eastern Syria. To the south, Syrian and Iran-backed forces are moving in on several fronts, as the United States supports its own coalition of rebel groups to get to the province first.

Victory for the Iran-backed force would give Tehran control of a large swath of the Syrian-Iraqi border, securing a land route through Iraq and across southern Syria to its proxy, ­Hezbollah, in Lebanon.

For the United States, control of Deir al-Zour brings a bargaining chip for the future and ­demonstrates to regional allies its willingness to challenge Iran, ­after Trump promised to roll back the country’s “rising ambition.”

“The weakening of ISIS was always going to open a race for territory, dominance and influence. The aggressive tone coming from Washington incentivizes Iran to speed up its operations,” said Emile Hokayem, a Middle East analyst at the International Institute for Strategic Studies. “The problem is that even what the U.S. sees as limited goals clash with more-ambitious Iranian ones.”

Proximity has sharpened tensions between the two sides. On Tuesday, the United States clashed directly with Iranian proxies for the second time in a month, bombing pro-government militia members advancing on an outpost used by U.S. Special Forces near Syria’s southeastern border with Iraq.

To reach the province, both sides are moving through Syria’s vast southern desert as they head for the Islamic State-held town of Bukamal, the first in a series of towns they must take in the push north to Deir al-Zour.

The race began last month after Russia, Turkey and Iran agreed to a cease-fire across parts of western Syria. Rebel commanders and Western diplomats  say the deal was intended to help the Syrian government and its allies concentrate resources in the east as they struggled to hold ground on multiple fronts.

“We see the link clearly now. Accepting those de-escalation zones meant the regime and its allies were able to relax and move resources,” said Abu Waleed, a commander with the U.S.-backed rebel group Usoud al-Sharqiya.

The retaking of the oil-rich Deir al-Zour region would diminish the government’s economic dependence on Iran and Russia, which have bankrolled its fight against the armed rebellion that began in 2011.

The U.S. military said last week that it had bolstered its “combat power” in southern Syria, warning that it viewed Iranian-backed fighters in the area as a threat to nearby coalition troops fighting the Islamic State.

U.S. Army Col. Ryan Dillon, a spokesman for the U.S.-led coalition in Iraq and Syria, told reporters last Thursday that Iranian-backed forces were 20 miles within a perimeter declared a week earlier in an attempt to de-escalate tensions and holding firm near the U.S. base at al-Tanf, a key border crossing.

Coalition aircraft, he said, dropped leaflets asking militia members to leave.

As Iraqi security forces fought pitched battles against Islamic State militants in Mosul in recent months, an array of Iran-backed and largely Shiite paramilitary groups have steadily pushed through the desert west of the city, reaching the Syrian border last week. 

Militia leaders said they aimed to move south along the border and retake the main crossing points into Syria.

Photographs recently circulated showed Maj. Gen. Qasem Soleimani, commander of Iran’s elite Quds Force, in the border area with Iraqi forces. Iraqi Prime Minister Haider al-Abadi has also visited the popular mobilization forces there and praised their “achievement” in reaching the frontier.

“At this stage, it depends less on what Assad and Iran does and more on what the United States does,” said Aron Lund, a fellow at the New York-based Century Foundation.

“If the U.S. and its allies have built up a strong enough force to move on al-Bukamal, then of course they can get there first. But can they get there in a way that is sustainable? They don’t seem to be sure of what they want to do.”

Zakaria Zakaria in Istanbul, Mustafa Salim in Baghdad, Dan Lamothe in Hawaii and Thomas Gibbons-Neff in Washington contributed to this report.