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.
What is the SDF?
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.
The SDF was "designed to facilitate recruitment among 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.
Why is US support growing?
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".
And US and SDF officials say post-IS Raqa will be administered by a civilian council of local residents.
But the issue is still likely to be among the top issues on the agenda when Turkish President Recep Tayyip Erdogan meets Trump in Washington on Tuesday.
How might Turkey react?
Turkey's concerns about the YPG were significant enough for Ankara to launch its own military operation inside Syria in August 2016, dubbed Euphrates Shield.
The operation had the dual goals of targeting IS and the Kurdish militia, particularly to prevent the YPG from controlling a contiguous strip of territory along the Syria-Turkey border.
While the Kurds have failed to link up the two "cantons" under their control in the northeast with the Afrin region to the west, the Turkish operation has largely floundered.
In April, Turkey launched raids on Kurdish positions in northeast Syria that killed at least 28 people, mostly YPG fighters, prompting a public expression of US concern.
Experts say Ankara lacks good options to prevent the SDF from continuing to lead the fight against IS, with international backing.
"Turkey has few options, other than to escalate," said Aaron Stein, resident senior fellow at the Atlantic Council's Rafik Hariri Center for the Middle East.
"They can invade Tal Abyad (on the Syria-Turkey border), bomb YPG, or move on Sinjar in neighbouring Iraq," he said.
"All of these options could slow the Raqa operation, but Ankara would then be placed in the untenable position of trying to slow a campaign to kill IS dudes. No one wants to be that country."