BEIRUT: With a string of victories over ISIS, Syria’s Kurds are proving themselves an ever more dependable ally in the U.S.-led fight against the extremists and building influence that will make them a force in Middle Eastern politics.

Aided by U.S.-led airstrikes, the Kurdish-led YPG militia may have dealt ISIS its worst defeat to date in Syria by seizing the town of Tal Abyad at the Turkish border, cutting a supply route to the jihadis’ de facto capital of Raqqa city.

The Kurds in northern Syria now control an uninterrupted 400 km stretch of the Turkish frontier, positioning them to help secure an area where the United States says more needs to be done to stop the flow of fighters joining ISIS.

The growing Kurdish sway is of no comfort to Turkey, worried by the risk of separatist sentiment among its own Kurds.

Ankara sees the widening Kurdish footprint as a possible threat to its border, and the YPG as little more than an offshoot of the PKK group which waged a decadeslong insurgency for Kurdish rights in Turkey. President Recep Tayyip Erdogan last week accused the West of helping “terrorists.”

But for now, the United States appears willing to risk upsetting its NATO ally to advance the goal of driving back ISIS in the Syrian half of its self-declared caliphate.

With Raqqa city just 80 km to the south, some analysts wonder whether Washington may now go even further by arming the YPG to help it wage war deeper into ISIS-held territory and to seal the border.

The United States and its allies are bombing ISIS positions in both Syria and Iraq. The coalition supports the government in Baghdad but is hostile to the government in Damascus, leaving it with few allies on the ground in Syria.

But even though that means the YPG is so far the only significant partner for the U.S.-led alliance in Syria, it says its request for arms has yet to be answered.

With or without new weapons, the YPG’s growing role will only strengthen its international standing and its influence over any eventual outcome of the Syrian war.

“It gives the Kurds legitimacy and enhances their bargaining position,” said Henri Barkey, a professor of international relations at Lehigh University in the United States.

With the onset of the Syrian uprising in 2011, the Syrian Kurds formed their own administration in three separate areas of northern Syria, coordinating with Damascus to expand into areas where the state was retreating. The capture of Tal Abyad means two of those regions are now geographically connected.

The Syrian Kurds say they do not aspire to found an independent state, but see their model of regional autonomy as one that could form the basis for an eventual settlement to the war now in its fifth year.

Erdogan has accused Syrian Kurds of displacing Arab and Turkmen residents from some areas they occupy, and some Arab refugees fleeing the area have complained of harsh treatment.

The YPG denies accusations of ethnic cleansing, and the U.K.-based Syrian Observatory for Human Rights, which monitors the conflict with a network of sources on the ground, says there is no evidence of systematic YPG abuses.

The YPG is confident it can secure new areas of the frontier near Tal Abyad: The Kurdish administration in the northeast has already managed to secure large sections of Syria’s border since early on in the crisis, said Redur Xelil, a spokesman.

“We are committed to protecting these borders because that way we are protecting our existence,” Xelil added. “We should be supported with weapons and equipment because the YPG has shown its effectiveness in expelling ISIS more than any other force.”

The YPG and the United States joined forces for the first time last year to repel an ISIS attack on the town of Ain al-Arab, known widely by its Kurdish name Kobani. The cooperation has deepened since then.

Backed by U.S.-led airstrikes, the YPG has driven ISIS from swaths of the northeastern province of Hassakeh – a region important in the fight against ISIS because it borders areas held by the group in Iraq.

Brig. Gen. Thomas Weidley, the chief-of-staff of the anti-ISIS coalition, Monday specifically praised the YPG.

“Since fall 2014, Kurdish forces in both Iraq and Syria, enabled by the coalition, have only taken territory from [ISIS], never ceding it,” he said in a post on Facebook.

But the Syrian Kurds complain that the support remains far below the level received by their Iraqi Kurdish cousins.

“If there is a decision to put pressure on Raqqa – the center of ISIS – this is a scenario where I can see the YPG getting more arms from [the United States],” Barkey said.

Though that moment still appeared far off, U.S. officials were likely planning for such a contingency, he added. “To me it is a logical next step.”

While they have proved effective fighting ISIS in Kurdish areas, it is uncertain how well the YPG would perform in Arab parts of Syria.

The Kurds’ attack on Tal Abyad – an ethnically mixed area – was aided by smaller Syrian rebel groups that are part of the more secular-minded opposition to President Bashar Assad.

In Hassakeh province, the Syrian Kurds still share control of two cities with the Syrian government. The sides have mostly left each other to their own devices.

But there are signs of strain. The Kurds’ alliance with Washington has fueled suspicions in Damascus of a conspiracy to break up Syria, while the Kurds are irritated by what they see as government attempts to recover lost influence in the region.

Tensions between the Syrian government and Kurdish security forces flared in the city of Qamishli overnight. Shots were fired, but no one was hurt, the Observatory and a Kurdish official said.

“The regime will with time get weaker,” said Nasir Haj Mansour, a Kurdish official in the northeast. “I do not imagine the regime will be able to strengthen its position.”