Turkish Tensions With Syrian Kurdish Fighters Strain Ties With U.S.

The new diplomatic effort came to a halt last week after Russian warplanes and Iranian fighters helped the Syrian military seize the advantage in the long-running battle for control of Aleppo, the country’s largest city. Amid the Assad regime’s advance, Turkey and Saudi Arabia pressed Syrian opposition leaders to withdraw from the talks in Geneva, according to diplomats and Syrian opposition leaders involved in the talks.

The rising role of the Kurdish fighters in Syria is creating new political strains, as well: Moscow, which is still seeking an apology from Turkey for shooting down a Russian jet last fall along the Turkey-Syria border, is trying to strengthen its ties with the Kurdish militants that Ankara views as a threat. The U.S., meanwhile, is trying to ensure the Syrian Kurds focus their firepower on Islamic State and don’t antagonize Turkey, which doesn’t want to see the Kurdish fighters receive international support for their hopes of one day creating their own state in the region.

More than 40,000 Syrians have been uprooted by the new fighting in Aleppo, with most of them trekking 60 miles north to the border with Turkey, where they are seeking refuge.

After successfully preventing the U.S.-backed Kurdish group from taking part in last week’s Geneva talks, Ankara is pressing its argument that the YPG—as the Kurdish fighting force is known—is an unreliable American partner in the fight against Islamic State.

For months, Ankara has complained to Washington that its Kurdish allies in Syria were smuggling U.S.-made weapons to PKK fighters who declared an end to a two-year-old cease-fire with Turkey last summer.

Since then, PKK militants have been embroiled in a worsening fight with the Turkish military that has transformed parts of southeastern Turkey into urban war zones. Kurdish insurgents have used homemade bombs, rocket-propelled grenades and automatic weapons to kill hundreds of Turkish security forces in recent months.

The tensions with Turkey come as the U.S. is increasing its cooperation with the Syrian Kurds by sending dozens of elite military advisers into northern Syria to work with YPG fighters and their Arab allies. That was evident when America’s key diplomat in the fight against Islamic State secretly traveled to northern Syria a week ago to meet with the Kurdish group to demonstrate the Obama administration’s backing.

During the trip, Brett McGurk, President Barack Obama’s special envoy for the global coalition to counter Islamic State, was photographed with a number of Kurdish militant leaders.

In one photo, a smiling Mr. McGurk was shown accepting a thank-you plaque from Polat Can, spokesman for the Syrian Kurdish fighters who first was a member of the PKK.

When photos emerged on social media showing a younger Mr. Can in a PKK uniform, critical Turkish media seized on it as clear evidence that the U.S. was working hand-in-hand with terrorists battling Turkish security forces in the streets of southeastern Turkey. American officials said they know Mr. Can as a YPG spokesman who has played a vital role in the fight against Islamic State.

For President Recep Tayyip Erdogan, the photographs, taken in the Syrian border town of Kobani, were an insulting public embrace of Turkey’s adversary, one that only reinforced his concerns that America is turning a blind eye to the threat posed by the YPG.

“How can we trust [you]?” Mr. Erdogan told reporters on his plane in remarks published Sunday. “Is it me that is your partner, or is it the terrorists in Kobani?”

As the fight with PKK militants in southeastern Turkey intensified late last year, Turkish officials provided the U.S. government with new evidence they said proved that Syrian Kurdish militants working with America had sent U.S.-made weapons and ammunition to PKK fighters battling Turkish security forces.

When Mr. Biden came to Istanbul, Turkish officials presented the U.S. with more complaints about the weapons flow.

“The Turks flagged concerns about cross-border weapons and ammunition transfers, and they shared some details about a few limited transfers they interdicted,” said one senior U.S. administration official who was present during the recent discussions with Mr. Biden. “But they provided no information—and we have no independent information—to indicate anything the United States has provided to groups fighting [Islamic State] in Syria has made its way into Turkey.”

Turkish officials expressed confidence in their intelligence showing Kurdish fighters smuggling U.S.-made weapons from Kobani, where American planes dropped weapons in late 2014 to help the Kurdish fighters drive Islamic State extremists out of the Syrian border town.

“Do they feel better when PKK kills Turkish police and soldiers with US-made weapons that YPG transfers from Kobani out of the batch they acquired from somewhere else, as opposed to the batch that was airdropped?” said one Turkish official.

While much of the weaponry used by the PKK is believed to come from Russia, U.S. officials said they have kept a close eye on the border to ensure that U.S. support for the Kurds in Syria doesn’t end up in Turkey. Turkey’s concerns are a main reason that the U.S. has restricted what it has directly given to the Kurdish fighters.

One senior administration official said the U.S. has a consistent message: Support for the Syrian Kurdish fighters isn’t an endorsement of the PKK.

“As the vice president made clear in Istanbul, the PKK is a terrorist organization, we condemn their attacks, we recognize the Turkish government’s right to self-defense, and we believe it is imperative for the PKK to cease violence as a first step back (to) a negotiated settlement,” he said.

Write to Dion Nissenbaum at dion.nissenbaum@wsj.com and Carol E. Lee at carol.lee@wsj.com