The Islamic State group may be on the run from its last bastion in Syria but the United States is gearing up for a longer stay in the country.

And if US forces are to counter Iranian and Russian influence as Syria struggles to emerge from civil war, they will need Turkey's help.

But Washington's relations with President Recep Tayyip Erdogan's testy government have been strained of late, posing a diplomatic challenge.

So when US President Donald Trump called his counterpart Friday, he needed to make a significant gesture -- and he seems to have delivered.

Tensions remain high, but Trump's apparent promise not to send any more weapons to the YPG Syrian Kurdish militia was a key concession to Ankara.

Without it, a major NATO ally might have moved closer to Iran and Russia, who are battling to save Bashar al-Assad's Syrian regime.

Some in Washington and the region may see it as a betrayal of a battlefield ally that was instrumental in capturing the IS capital Raqa.

But the Kurds now have the strength to hold their own in northeastern Syria, and Washington is turning to the bigger picture.

"We can't operate in the long term in Syria without Turkish bases and Turkish airspace and to some degree Turkish diplomatic support," said former US ambassador James Jeffrey.

"So that's what the call is all about," Jeffrey, a former senior US national security adviser and envoy to both Ankara and Baghdad, told AFP in Washington.

Syria has been ensnared in civil war between Assad and an array of armed groups since 2011, and the chaos allowed IS to seize part of the east.

US commanders mobilized a coalition -- including the YPG -- to take on the jihadists, and the Kurdish Syrian fighters were in the vanguard when Raqa fell last month.

Turkey, a NATO ally, was nominally part of the US-led coalition, but in led its own intervention into northern Syria, battling extremists but also the Kurdish forces.

The YPG is an offshoot of the same movement as the Kurdistan Workers' Party (PKK), which is waging a separatist insurgency inside Turkey.

- Driving Erdogan crazy -

According to Jeffrey, who as a fellow of the Washington Institute for Near East Policy is still in touch with senior Turkish figures, "nothing drives Erdogan more crazy" than US backing for the YPG.

But at the same time, Ankara and Washington share an interest in countering Russia and Iran's influence in Syria and in shaping the country's future as peace talks loom.

After Friday's call, Turkey's Foreign Minister Mevlut Cavusoglu said Trump had promised arms supplies would halt and "essentially he said this nonsense should have been ended earlier."

The White House was less explicit, but confirmed Trump "informed President Erdogan of pending adjustments to the military support provided to our partners on the ground in Syria."

The White House also underlined that both leaders had discussed the importance of the upcoming United Nations-backed Syrian peace talks due to start next week in Geneva.

This is important because Turkey is now also party to a parallel and potentially rival political process being conducted under Russian auspices in the Kazakh capital Astana.

On Wednesday, Erdogan met presidents Vladimir Putin of Russia and Hassan Rouhani of Iran -- Assad's key allies -- in the Russian resort of Sochi, to discuss both sets of talks.

This could explain the timing of Friday's call, explained former Turkish member of parliament Aykan Erdemir, in an interview with AFP.

"There seems to be a greater convergence between NATO ally Turkey's Syria policy and Russia and Iran's Syria policy," said Erdemir, a scholar at US think tank the Foundation for Defense of Democracies.

In the early years of the Syrian conflict, Erdogan like then US president Barack Obama was rhetorically a staunch foe of Assad, despite frequent US-Turkish diplomatic spats.

But Russia's dramatic military intervention and steadily more powerful Iranian support saved the Syrian strongman's regime, and both Ankara and Washington confront a new reality.

For Erdemir, Erdogan is edging towards accepting that Assad's regime will cling on in some form, and is seeking a tactical accommodation with Moscow and Tehran.

He noted Turkey had allowed Assad to fly across its airspace for talks with Putin in Russia -- while Russian military transports brought war supplies in the other direction.

"Erdogan recently referred to Assad as the central administration, the central government in Syria, and many saw that as a landmark statement," he said.

- Radical U-turns -

But he predicted the shift would not last; Erdogan has much invested in painting Assad as a war criminal, and Russian forces will eventually want to edge Turkish troops out of Syria.

"I have serious doubts about the sustainability of Erdogan's game with Iran and Russia," he said, describing Turkish foreign policy in recent years as "a series of radical U-turns."

It might not yet be a U-turn, but after Trump's called Jeffrey said: "Are we better off now than we were 24 hours ago with the Turks? Probably."

Last week, US Defense Secretary Jim Mattis stirred surprise when he announced US military presence in Syria would remain even after the Islamic State's defeat.

"We are going to make sure we set the conditions for a diplomatic solution," he said.

The solution Washington seeks would ideally lead to an elected government -- without Assad -- in Damascus, despite the effort invested by Iran and Russia to protect him.

But if that proves impossible, the priority will be stability and curtailing Iran's growing power.

To wield influence, the US will need the longer-standing military presence that Mattis envisages -- and to maintain that, Turkish support.

Turkey, for Jeffrey, is posing Washington a legitimate question: "We want a policy. What is your policy?"

Washington is now, perhaps, moving closer to an answer.