Fighters of the Kurdish People's Protection Units (YPG) carry their weapons at a military training camp in Ras al-Ain, February 13, 2015. Rodi Said/Reuters The Pentagon is weighing a new request from Turkey to train and equip Arab rebels battling government forces in northern Syria, The Wall Street Journal reported on Wednesday.

The request, which reportedly came just one week before an ISIS-linked suicide bomber killed 10 people in Istanbul, is evidently an attempt to seal a vulnerable stretch of the Turkish-Syrian border that continues to serve as a transit point for foreign fighters and weapons.

It is also an attempt to appease Turkey, which has expressed concern to Washington that the Kurdish People's Protection Units (YPG) — a militia linked to Turkey's longtime enemy, the Kurdistan Workers' Party (PKK) — is taking advantage of its anti-ISIS partnership with the US to gain power and territory along the Turkish-Syrian border.

A newly empowered, US-backed Arab rebel brigade aimed at enabling larger groups of Arab forces fighting in Syria would presumably serve as a counterweight to Kurdish territorial ambitions in the north. That's according to Aaron Stein, a Turkey expert and senior resident fellow at the Atlantic Council who spoke with Business Insider on Wednesday.

That, in turn, would allow the US-led anti-ISIS coalition to stem the YPG's advances — a primary concern for Turkey — without sacrificing ongoing efforts to seal off Turkey's southern border to jihadists.

"Whether the YPG will actually listen to the US is a different story. But the US is effectively telling the YPG to observe Turkey's red line," Stein said, referring to the Turkish leadership's insistence that the Kurds remain east of the Euphrates.

Kurdish People's Protection Units (YPG) fighter walks near residents who had fled Tel Abyad, as they reenter Syria from Turkey after the YPG took control of the area, at Tel Abyad town, Raqqa governorate, Syria Thomson Reuters Wladimir van Wilgenburg, a Kurdish affairs expert embedded in Iraqi Kurdistan, largely echoed those sentiments.

"Most likely the Turkish plan is to insert these forces into the Azaz border strip and to prevent YPG expansion into these areas in the future," van Wilgenburg told Business Insider on Thursday, referring to the northern Syrian town of Azaz through which Turkey funnels weapons and supplies to the rebels it supports in Aleppo.

He added: "Turkey wants to prevent Kurdish expansion and stop them from linking the Kurdish administrations in Afrin and Kobani."

"Of course, the YPG will not be happy about this," van Wilgenburg added, partly referring to the mutual distrust and tension that has often flared up between Kurdish and Arab factions in Syria. "But they also need the US."

It is true that the Kurds have relied on their partnership with the US to foment a degree of legitimacy that they have been denied in the past. But the entry of Russia into the Syrian conflict has presented the Kurds with a new option, should the US attempt to halt the YPG's momentum and empower Arab forces.

"We welcome a strategic relationship with both the US and Russia,” Sherzad Yazidi, a representative of the Rojava administration living in Sulimaniya, told Politico in November. “One wouldn’t be at the expense of the other.”

Still, Turkey is already beginning to worry about the "nightmare scenario" of Russia supporting the Kurds in Syria and facilitating their expansion westward — especially since Russia is still looking for ways to retaliate against Turkey for shooting down its warplane in November.

A fighter from the Democratic Forces of Syria takes an overwatch position at the top of Mount Annan overlooking the Tishrin dam, after they captured it on Saturday from Islamic State militants, south of Kobani, Syria, December 27, 2015 REUTERS/Rodi Said "One worry in Ankara since the diplomatic crisis with Moscow [in November] has been Russian support for the [Kurdish] PYD and, in particular, a possible PYD movement toward the west of the Euphrates with Russian encouragement and air support," Merve Tahiroglu, a research associate focusing on Turkey at the Foundation for Defense of Democracies, told Business Insider last month.

Indeed, the Turkish pro-government daily newspaper The Daily Sabah reported in early December that Russia was delivering weapons and heavy armor to the YPG while instructing them to attack opposition groups, many of whom are backed by Turkey.

But Stein, for his part, isn't convinced that a new training initiative focusing on Arab fighters would drive the Kurds into Russia's arms.

"I don’t think the Kurds will think much about it," he said. "They’re completely self-interested actors who are moving to establish an independent structure inside Syria, known as Rojava. Above all, they are intent on realizing that goal."

That said, however, the program itself is at least partially aimed at curbing Kurdish ambitions. As such, if it were to pan out, the plan could feasibly reduce the Kurds' confidence that their battlefield victories will one day yield political recognition from the US. This, in turn, might lead them to look for new partners.

Van Wilgenburg put it bluntly.

"Russia would be happy were any problems to arise between the Kurds and the US," he said.