The US-backed Syrian Democratic Forces militia is set to lead the fight for the Islamic State group's bastion of Raqa, but its role has stoked tensions between Washington and Turkey.
The alliance of Arab and Kurdish fighters has advanced to within a few kilometres (miles) of Raqa on several fronts, and last week captured the strategic town of Tabqa and the adjacent dam from the jihadists.
However, Ankara considers the key Kurdish component of the SDF to be an affiliate of the designated "terrorist" group Kurdistan Workers' Party (PKK), which has waged a decades-long insurgency inside Turkey.
The alliance was formed in October 2015 in part to address Turkey's concern about the rising profile of the Kurdish People's Protection Units (YPG) militia.
The militia had won a string of victories over IS with the US-led coalition's support, including the highly symbolic recapture of the border town of Kobane in January 2015.
But Ankara considers the YPG to be the Syrian branch of the PKK, and was increasingly unhappy about the swathes of territory coming under YPG control.
The advances were also stirring local tensions with Syrian Arabs concerned that Kurdish forces were seeking to dominate non-Kurdish regions.
Arabs and provide an additional nominal degree of separation between US support and the PKK", the International Crisis Group (ICG) think tank said in a report last month.
But the SDF's ranks, which estimates place at anywhere between 25,000 and 45,000 fighters, remain dominated by the YPG, which also retains control of the alliance's command, analysts say.
Since its formation, the SDF has won multiple victories against IS, and in November 2016 it announced the start of a long operation to oust the jihadists from Raqa.
In the months since, it has gradually closed in on Raqa, working to encircle it before launching a final assault, expected to start next month.
With the formation of the SDF, Washington began channeling more direct support to the anti-IS fighters on the ground in Syria.
But it continued to insist that supplies including armoured cars went to the Arab components of the alliance, not the YPG.
Last month however, President Donald Trump's administration announced that it would for the first time directly arm the YPG elements of the alliance.
"The US administration appears to have concluded that the benefits of driving IS from Raqa as soon as possible justify the potential costs of further damaging Washington's strategic alliance with Ankara, and the risks associated with attempting to seize an overwhelmingly Arab city... With a Kurdish dominated force," the ICG report said.
Washington has sought to placate Turkey by saying that the weapons will be metered out carefully, and insisting it still wants to "work with the Turks... To take Raqa down".