Turkey threatens Syria's Kurds: A headache for the US?

The United States has scrambled to contain the tensions, as it seeks to retain the YPG as a key partner in its battle against a resilient Islamic State group.

How serious are threats?

Since 2016, Turkey has carried out two operations against Kurdish forces in Syria, the last of which saw Ankara-backed Syrian rebels seize the northwestern enclave of Afrin in March.

Erdogan has since repeatedly threatened to march east into more Kurdish-held territory, but analysts say the timing adds weight to the latest warnings.

Turkey brokered a deal with Russia in September to stave off a regime attack on the northwestern rebel bastion of Idlib, thus freeing it up to set its sights on Kurdish-held territory further east.

On the world stage, Ankara is feeling emboldened and seeks to score diplomatic points as Saudi Arabia, a rival regional heavyweight, grapples with global outrage over the murder at the Saudi consulate in Istanbul of journalist Jamal Khashoggi.

And Turkey has seen its relationship with NATO ally Washington improve after it freed American pastor Andrew Brunson from detention last month.

With shelling east of the Euphrates River, analysts say Erdogan is testing the waters, specifically to see how the United States will react.

"He is trying to see how far he can go with military action in the areas east of the Euphrates before the US responds negatively," said Nicholas Heras, an analyst at the Center for a New American Security.

What can US do?

Turkey views the YPG as "terrorists", but for the United States they are a key ally in its fight against IS jihadists.

The YPG has spearheaded a Kurdish-Arab alliance, backed by the US-led coalition, that has pushed back the extremists from Syria's northeast.

But the battle is not yet over, and the Syrian Democratic Forces (SDF) alliance is still fighting the jihadists in the country's far east near the Iraqi border.

In response to Turkish shelling, the Kurdish-led SDF last Wednesday said it had temporarily halted its offensive against the jihadists in the eastern province of Deir Ezzor.

"The United States is stuck in the middle of all of this, wanting only to push the anti-IS offensive to a conclusion on schedule," said Aaron Lund of The Century Foundation.

"For the Syrian Democratic Forces, this must seem like a rare source of real leverage over the superpower," Lund said.

The day after the SDF announced it was suspending fighting, Turkish and US troops began joint patrols on the outskirts of the flashpoint city of Manbij.

They had been laid out as part of a "roadmap" reached by the NATO allies in June to avoid a clash, and under which YPG forces were to withdraw from the city.

On Friday last week, US forces started patrols in Kurdish-held areas along the Turkish border, sparking criticism from Ankara.

Will fight against IS suffer?

Syria expert Fabrice Balanche said Washington was in a bind.

"If the US give in to Turkey, they will no longer be able to count on the Kurds" to fight IS, he said.

The SDF has yet to announce a resumption of the military operation it launched in September to expel IS from its last holdout on the Iraqi border.

Kurdish affairs expert Mutlu Civiroglu said the pause in fighting sent "a clear message to the international coalition".

The SDF is saying, "We're partners, and when I am facing threats like this you have to stop it," he said.

Many fighters on the Deir Ezzor front hail from Kurdish-held towns on the Turkish border, Civiroglu said.

"Their homes, their families are under attack," he said. For them, "it's hard to focus on the fight -- already a very tough fight."

Analysts say Turkey's threats are only one of many hurdles to defeating the jihadists, who launched a deadly counter-attack during sandstorms late last month.

Current tensions could draw out the battle against IS in far eastern towns, including Hajin, but are unlikely to reshape the battlefield.

"IS is militarily defeated even though it's putting up a fight in Hajin," said Aaron Stein, a resident senior fellow at the Atlantic Council.

"The US will get it done, eventually."