TALL ADH-DHAHAB, Syria—A convoy of armored vehicles flying American flags, machine gunners perched atop their tall metal frames, rumbled into this village of dirt roads and concrete houses in northeast Syria last week.

They are a familiar sight in the area. Over scalding hot glasses of tea, U.S. Army officers and their allies in the Kurdish-led Syrian Democratic Forces met with local men to ask about security threats now that Islamic State forces are in retreat.

But the villagers had a question of their own: Would the Americans stay?

After the chaotic withdrawal of the last U.S. forces from Afghanistan, leaders in this corner of Syria say they fear a departure of the roughly 900 American soldiers, backed by U.S. air power, who patrol areas held by an American-allied militia and, they say, are essential to protecting them from hostile forces.

“If they withdraw, the next day we’ll be attacked from all sides, the regime from this way, Turkey from that way,” said Benkin Hussein, 45, an unemployed man in Tall Adh-Dhahab.

A man met with U.S. Army soldiers over tea in the garden of a house in Tall Adh-Dhahab last week.

Col. Wayne Marotto, spokesman for the U.S.-led military coalition against Islamic State, gave an interview to local Syrian television at the RLZ outpost last week.

After a decade of war sparked by a popular uprising, Syria is now fractured among an array of forces and outside powers including Russia and Iran, who provide military support to Syrian President Bashar al-Assad. Turkey supports armed rebel groups. Islamic State fighters also remain active.

During a visit by a Wall Street Journal reporter to Iraq and northeast Syria, American officers made the case that the U.S. presence is critical to keeping pressure on Islamic State insurgents and supporting U.S. allies in both countries.

Others argue it is time to go. Robert Ford, the former U.S. ambassador to Syria, has argued that since the Kurdish-led region in Syria cannot support itself, American interests would be better served by withdrawing U.S. troops to focus on other priorities and relying on Turkey and Russia to fight Islamic State.

The Biden administration, following moves by former President Donald Trump, has said it is determined to reduce the American military footprint in the broader Middle East, nearly 20 years after the Sept. 11, 2001, attacks on New York and Washington that prompted the invasion of Afghanistan.

In Iraq, which the U.S. invaded in 2003, the White House has already committed to pull out combat troops by the end of the year, though forces providing training and support to Iraqi security forces—some 2,500 troops at present—will remain on the ground.

Military officers on the ground in Iraq and Syria say they are committed to staying in the region and that the Afghan withdrawal hasn’t affected their mission.

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“We’re still here, still partnering with them and showing that commitment,” said Col. Wayne Marotto, the Baghdad-based spokesman for the U.S.-led military coalition against Islamic State, who visited U.S. forces in Syria last week.

But, Col. Marotto said: “You and I can’t tell the future on what’s going to happen with the next politician and his policies.”

U.S. officials have reached out to American-backed forces in Syria in recent days trying to reassure them that the U.S. is staying in the region. Acting Assistant Secretary of State for Near Eastern Affairs Joey Hood spoke with SDF officials late last month “to reiterate the U.S. commitment to the campaign against ISIS and stability in the region,” according to a tweet from the account of the U.S. Embassy Syria.

The RLZ outpost is populated by National Guard members from Alabama.

RLZ is a camp of low buildings, a landing strip and prefabricated housing units.

The U.S. currently maintains a series of small bases dotting the desert in northeast Syria aimed at preventing any resurgence by Islamic State. They include an outpost known as RLZ, a camp of low buildings, a landing strip and prefabricated housing units that resemble shipping containers. Responsible for securing a corner of Syria near the border with Iraq, the base is populated by National Guard members from Alabama.

U.S. air power and other military support is also an important source of stability for the Iraqi security forces, who crumbled when Islamic State seized a vast section of the country in 2014. U.S. officials argue that a collapse of that scale is unlikely to happen again, but Baghdad’s forces are under pressure from both Islamic State insurgents and Iranian-backed militias.

The American military presence also creates risks for the government in Baghdad, however, where the parliament voted to expel American forces last year.

In 2019, Mr. Trump decided to pull U.S. troops from Syria before reversing his decision weeks later. In the interim, Turkey launched a military operation against American-allied Kurdish militants, who Ankara considers to be a security threat within its own borders.

An oil-and-gas facility seen from a U.S. armored vehicle during a patrol in northeastern Syria last week.

A member of the Kurdish-led Syrian Democratic Forces at a plant in northeastern Syria last week.

Russia also sent its armed forces to the same northeastern corner of Syria to monitor a truce agreement, triggering a series run-ins between Russian and American patrols last year, including a collision that injured four U.S. soldiers.

The biggest player is the Syrian regime in Damascus. Backed by Moscow and Iran, it has vowed to retake parts of Syria that it lost following the 2011 rebellion against Mr. Assad, including areas in northeast Syria held by the U.S.-backed SDF.

The U.S.’s chief concern is any sign of a revival of Islamic State. Once the dominant power across much of Syria and Iraq, it persists in both countries as an insurgent group and recently claimed responsibility for suicide bombings in Baghdad that killed dozens of people.

It also claimed credit for its Afghan branch’s bombing at the Kabul airport that killed around 200 people on Aug. 26.

U.S. Army soldiers prepared to go on patrol from RLZ. U.S. officials say that patrols play a role in deterring rivals in the region.

U.S. soldiers in northeastern Syria keep in regular contact with residents as they try to track militant activity. After visiting a village last week, patrolling U.S. soldiers also dropped in on an oil and gas plant, talking with local SDF commanders amid clouds of black smoke and puddles of crude oil on the ground.

“We’re solely focused on their security,” said Capt. Freddy Taul, who led a convoy of mine-resistant vehicles into the area.

But U.S. officials acknowledge that American military patrols also play an important role in deterring other rivals in the region such as Iran and its allied militias, which command tens of thousands of fighters in Iraq and Syria and are a growing threat.

“I would say it’s a signal to all the malign actors and adversaries here in the region,” said Col. Marotto of the patrols.

The view from a helicopter during a flight from Erbil air base in northern Iraq to northeastern Syria.

Iranian-backed paramilitary groups escalated assaults on U.S. bases in both Iraq and Syria earlier this year, launching missile and drone attacks that have killed and injured U.S. personnel and civilians. The militias have sworn to drive the U.S. from the region, intensifying their campaign following the Trump administration’s killing of top Iranian general Qassem Soleimani in January 2020.

American forces have also had to adjust to a new threat in the form of cheap Iranian-made drones, which have repeatedly attacked U.S. bases since March.

The U.S. military has used air defenses and flown drones into areas of Iraq not normally patrolled by the coalition to put pressure on the militias, a senior military official said.

Lt. Col. Scott Desormeaux, a Louisiana National Guard commander who leads forces in both Iraq and Syria from the Erbil air base in northern Iraq, compares the drones to the spate of roadside improvised explosive devices, or IEDs, that plagued American forces at the height of the insurgency following the 2003 invasion of Iraq.

“I put people specifically on the problem and said, ‘Your job is to solve how we’re going to get quicker to the kill,’” he said.

Leaders in northeastern Syria say they fear a departure of U.S. soldiers there.

Write to Jared Malsin at jared.malsin@wsj.com