Obama administration officials acknowledged the PKK and YPG have links and coordinate with each other in the fight against Islamic State, but they said the U.S. continues to formally shun the PKK while dealing directly with YPG. The groups operate under separate command structures and have different objectives, the officials said.

“We recognize YPG are fighting [Islamic State] and that Americans are giving support to it,” a Turkish foreign ministry official said. “We transmit our views to American allies.”

Constantly shifting alliances in the region mean the PKK’s rise isn’t certain to continue. But the guerrilla group’s growing stature has alarmed Turkey, a crucial North Atlantic Treaty Organization ally of the U.S., with whom the PKK has fought a three-decade war costing some 40,000 lives. The PKK is in peace talks with Turkey, and a political party linked to the PKK won a record 12% of the vote in Turkey’s June parliamentary elections. Troubled by the PKK’s battlefield victories, Ankara has vowed to prevent the formation of a Kurdish state in Syria.

U.S. defense officials said coordination with YPG units, including some inside Syria, has improved the ability of coalition aircraft to strike Islamic State positions and avoid civilian casualties. U.S. Defense Secretary Ash Carter during a visit to the region this week said YPG forces in Syria are “extremely effective on the ground.”

By contrast, Ankara agreed only on Thursday to allow coalition airstrikes from an eastern-Turkey air base , after months of negotiations in which President Recep Tayyip Erdogan’s government resisted international calls to enter the war with Islamic State. U.S. officials said the base deal shouldn’t affect U.S. air support to Kurdish fighters in Syria and may help increase collaboration with the YPG because jets and drones will be closer to the battlefield.

“There’s no reason to pretend anymore,” said a senior Kurdish official from Kobani. “We’re working together, and it’s working.”

U.S. war planners have been coordinating with the Syrian affiliate—the People’s Defense Units, or YPG—on air and ground operations through a joint command center in northern Iraq. And in two new centers in Syria’s Kobani and Jazeera regions, YPG commanders are in direct contact with U.S. commanders, senior Syrian Kurdish officials said.

The PKK and its Syrian affiliate have emerged as Washington’s most effective battlefield partners against Islamic State, also known as ISIS, even though the U.S. and its allies have for decades listed the PKK as a terrorist group. The movement in the past has been accused of kidnappings, murder and narcotics trafficking, but fighters like Ms. Ruken have presented the world an appealing face of the guerrillas—an image of women battling as equals with male comrades against an appallingly misogynist enemy.

Ms. Ruken’s journey provides a glimpse behind the remarkable rise of the Kurdistan Workers’ Party, or PKK, the cultlike Marxist-inspired group she fights for and whose triumphs against Islamic State have helped it evolve from ragtag militia to regional power player.

She has deployed to reverse their advances on self-governing Kurdish communities. Last summer, she says, she helped rescue Kurdish-speaking Yazidis besieged on Sinjar Mountain. Her unit has fought Islamist insurgents and conventional armies in Syria, Turkey, Iran and Iraq—countries where an estimated 30 million Kurds live.

Now the 24-year-old is a battle-hardened guerrilla, using machine guns and rocket-propelled grenades to fight Islamic State extremists in Syria and Iraq.

SINJAR MOUNTAIN, Iraq—Nine years ago, Zind Ruken packed a bag and left her majority-ethnic-Kurdish city in Iran, escaping a brutal police crackdown and pressure to marry a man she’d never met.

2

Mosul

Raqqa

Aleppo

Erbil

4

Sinjar

Mountain

Kirkuk

syria

Sanandaj

iraq

iraN

50 miles

Ramadi

50 km

Baghdad

People’s Defense Units (YPG) — Syria

People’s Defense Forces (HPG) — Iraq

Party of Free Life of Kurdistan (PJAK) — Iran

Kurdistan Workers’ Party (PKK)

turkey

Qandil Mountain

Kobani

PKK headquarters

People’s Defense

Forces (HPG)

People’s Defense

Units (YPG)

Aleppo

Mosul

Erbil

Sinjar Mountain

Raqqa

50 miles

Sanandaj

Kirkuk

Prominent

50 km

syria

PKK base

Euphrates R.

Tikrit

Party of Free Life

of Kurdistan (PJAK)

Tigris R.

Damascus

iraN

iraq

Ramadi

Baghdad

Sources: CIA; Institute for the Study of War

Kurdistan Workers’ Party (PKK)

turkey

Kobani

Qandil Mountain

PKK headquarters

People’s Defense

Forces (HPG)

People’s Defense

Units (YPG)

Aleppo

Mosul

Raqqa

Erbil

Sinjar Mountain

Prominent PKK base

50 miles

Kirkuk

Sanandaj

50 km

syria

iraq

Euphrates R.

Party of Free Life

of Kurdistan (PJAK)

Tikrit

Tigris R.

iraN

Damascus

Ramadi

Baghdad

Sources: CIA; Institute for the Study of War

America’s association with a terror-listed Maoist-inspired militia, even if indirect, shows how dramatically Syria’s conflict has reconfigured regional alliances and eroded once-rigid borders.

Just two years ago, President Barack Obama told Turkey the U.S. would continue to aid its battle against PKK “terrorists.” The U.S. continues to share intelligence about the PKK with Turkey, and military officials from the two countries sit together in an Intelligence Fusion Cell in Ankara established by the George W. Bush administration to help Turkey fight the group.

But now, “the U.S. has become the YPG’s air force and the YPG has become the U.S.’s ground force in Syria,” said Henri Barkey, a former State Department analyst on Turkey now at Lehigh University.

Some senior U.S. and British diplomats said the time has come for the U.S. and some European states to consider a broader rapprochement with the PKK. But U.S. officials said Washington is unlikely to revise the PKK’s terror listing without a green light from Turkey, which has itself sent mixed messages to Washington about its own dealings with the group.

U.S. military personnel aren’t on the ground inside Syria vetting Kurdish forces, making it difficult to discern the affiliations of individual Kurdish fighters who may benefit from U.S. airstrikes, said a senior U.S. defense official. “These guys don’t exactly wear patches identifying what groups they’re fighting for,” the official said, “but they are fighting the right guys.”

The PKK says its affiliates—Syria’s YPG and groups called the PJAK in Iran and the HPG in Iraq—are separate but closely linked. PKK fighters and some analysts say they are one and the same.

“It’s all PKK but different branches,” Ms. Ruken said, clad in fatigues in her encampment atop Sinjar Mountain this spring as a battle with Islamic State fighters raged less than a mile away at the mountain’s base. “Sometimes I’m a PKK, sometimes I’m a PJAK, sometimes I’m a YPG. It doesn’t really matter. They are all members of the PKK.”

On the battlefield, fighters like Ms. Ruken have the momentum. Since the Syrian uprising flared in 2011, the PKK and YPG have seized and defended large swaths of oil-rich territory in Syria and Iraq and are busy building state institutions. U.S. airstrikes last year helped the YPG repel an Islamic State onslaught on the Kurdish city of Kobani.

In June, the fighters captured the Islamic State stronghold of Tal Abyad, supported by U.S. air power, connecting long-disjointed Kurdish regions and dramatically expanding the territory they control.

Wildflowers cover alluvial plains at the foot of Sinjar Mountain, to which thousands of Yazidis were forced to flee under attack by Islamic State militants who had singled them out for murder and enslavement. PKK guerrillas rescued many Yazidis by creating a humanitarian corridor through the militant positions. Photo: Erin Trieb for The Wall Street Journal

‘We’re not terrorists’

“People look at us as if we’re terrorists and they put us on this blacklist. We’re not terrorists,” said Ms. Ruken, who like all PKK fighters uses a nom de guerre—hers means “alive smiling”—and declined to give her real name. “The Kurds know what we are fighting for. They know we will give our souls for them.”

The Kurdish guerrilla groups pledge allegiance to Abdullah Ocalan, the PKK chief imprisoned on a Turkish island since 1999. From jail in 2005, he established PKK affiliates that evolved into today’s YPG, HPG and PJAK.

The PKK and affiliates have car-bombed Turkish cities, kidnapped hundreds and killed Turkish and Kurdish state employees. In 2009, the U.S. Treasury Department designated their leadership as significant narcotics traffickers. The PKK ruthlessly dispatches Kurdish political rivals in Syria and elsewhere, according to New York based Human Rights Watch.

Zagros Hiwa, a PKK spokesman, said: “We have been defending our people against the denial and elimination policies of the Turkish state against the Kurds. Our struggle has always been on the basis of legitimate self-defense.”

The PKK practices an offshoot of Marxism it calls Democratic Confederalism. The group’s utopian goals echo those of some Cold War-era leftist militias. It aims to create a Maoist-inspired agrarian society that opposes landowning classes, espouses gender equality and distances itself from religion. Its guerrillas speak of a leaderless society of equals but also glorify Mr. Ocalan with fanatical devotion. They talk of needing to inculcate Kurdish populations with their ideology, rigidly centralized around Mr. Ocalan’s writings.

The group’s largely pro-West stance, and its deployment of female fighters like Ms. Ruken, has brought sympathy from Western governments and populations. Hundreds of volunteers from the U.S. and Europe have enlisted with the group since 2014.

Calls are growing from European and some U.S. policy makers for the PKK to be removed from terror lists and directly receive arms from Washington. In February, two fighters from the YPG’s all-female YPJ militia were invited to Paris’s Élysée Palace to meet with President François Hollande —their first such meeting with a NATO leader.

“The Kurds have emerged as the best buffer against Islamic State, and the PKK’s military prowess has shifted perceptions of them in the West,” said Marc Pierini, former European Union ambassador to Turkey now at the Carnegie Endowment in Brussels. “It looks like their moment may be coming.”

Female PKK fighters greet male counterparts before attending a meeting at the operations base on Iraq’s Sinjar Mountain. Photo: Erin Trieb for The Wall Street Journal

But Ankara, which relaunched peace talks with the PKK in 2012, is nervous its advances and burgeoning links with the West will strengthen its negotiating position, said Western diplomats and analysts. And the PKK’s expanding strength comes amid a rising tide of Kurdish autonomy that could augur a push for Kurdish independence across the Middle East, deepening the region’s fault lines.

At the PKK’s Qandil Mountain base in Iraq, the group’s chief commander, Cemil Bayik, said in an interview that perceptions of the PKK were shifting dramatically. “Islamic State’s attacks on the Kurds, and the Kurds fighting back against Islamic State, has changed the international attitude toward all Kurds, especially the perception of the PKK,” he said. “Now I want to ask: Who are the terrorists?”

Around the base’s cluster of buildings, fighters with AK-47s patrol in baggy Kurdish shalwar pants. The winding road there snakes past a massive color image of the imprisoned Mr. Ocalan etched into the mountainside, maintaining vigil on the soldiers below.

Ms. Ruken’s war

Fighters like Ms. Ruken trace the arc of a Kurdish militia expanding its sway across these troubled borderlands. While her tale isn’t independently verifiable, interviews with other footsoldiers like her echo elements of her story.

In 2006, aged 15, she resolved to join the PKK after Iranian security forces broke up her family’s New Year celebrations, beating and arresting her mother, father and older brother. Their crime: celebrating with a traditional Kurdish bonfire while clad in traditional Kurdish dress.

“That made a fire inside me,” said Ms. Ruken, whose ginger-colored hair sets her apart. “I couldn’t accept it.”

She joined an underground Kurdish women’s group with PKK links in her northern-Iran hometown of Sanandaj, training for two years in small arms and light artillery. She then traveled to Mr. Bayik’s Qandil Mountain base, the heart of the group’s operations in exile.

Fighting with the PKK meant abandoning personal identity and accepting extreme austerity. Ms. Ruken and her comrades go by battlefield names chosen to honor fallen friends or convey political convictions. They are forbidden to own property, have romantic relationships or speak much of their pre-PKK past.

The fighters often use a vocabulary of Marxist revolution honed in obligatory study of Mr. Ocalan’s writings. Stories of personal sacrifice are often so extreme as to seem exaggerated.

“We are not fighting just for ourselves,” said Chavon Ageet, a fighter in Ms. Ruken’s unit who chose his name, meaning “sheep herder,” after a fallen friend. “If any Kurd fights only for their own family, we will never have our own Kurdistan.”

“We need to establish the greater Kurdistan first,” said Mr. Ageet, adding that he regularly fights under the command of women, “and then think about marriage.”