Turkey said on Tuesday that it would begin its long-threatened offensive "soon", after Trump gave what was widely seen as a green light at the weekend, ordering the pullback of US troops who had previously served as a buffer.

Trump has blown hot and cold since his surprise announcement, which drew a storm of criticism even from within his own party.

But despite the mixed messages, the Turkish government sent more troops to the border overnight.

AFP correspondents witnessed the arrival of a large convoy of vehicles in the Turkish border town of Akcakale.

The Kurdish authorities in northeastern Syria called up civilians on Wednesday in readiness for the planned assault.

"We announce three days of general mobilisation in northern and eastern Syria," they said, urging all civilians to "head to the border with Turkey... to resist during this delicate historical moment".

They said they would hold their erstwhile US ally and the whole international community responsible for any "humanitarian catastrophe" that unfolds.

Turkey says it wants to establish a "safe zone" on the Syrian side of the border where it could send back some of the 3.6 million refugees it hosts from the eight-year civil war.

But the Kurds say Ankara's real goal is to dilute their demographic dominance of the northeast with an influx of mostly Sunni Arab refugees from other parts of the country now living in southwestern Turkey.

Ankara has strongly opposed Washington's support for Kurdish forces in Syria citing their links to the outlawed Kurdistan Workers' Party (PKK) which has fought a deadly insurgency against the Turkish state since 1984.

'We will not leave'

In the border town of Ras Al-Ain, Kurdish-led security forces set up new checkpoints and stockpiled tyres to set alight to blur the vision of Turkish military pilots, an AFP correspondent reported.

Ras al-Ain was one of the places from which US troops withdrew on Monday.

Kurdish authorities in the town called for protesters to gather at the border later on Wednesday.

Kurdish flags were hoisted on tents, opposite the Turkish flag flying on the other side of the frontier.

"We will not leave this land," said Kawa Sleem, a 32-year-old Ras al-Ain resident.

"War has been chasing us for years, and everyday (Turkish Prime Minister Recep Tayyip) Erdogan threatens us with a new attack," he added, pledging to defend the area with "all means available."

Ras al-Ain is one of the first areas on which Ankara's planned offensive is expected to focus. Like Tal Abyad, further west, its population is mainly Arab.

Kurdish forces have dug trenches and tunnels in both areas, covering streets with metal canopies to block the cameras of Turkish drones.

Turkish Defence Minister Hulusi Akar said the military build-up along the border was continuing on Wednesday.

"Our preparations regarding the operation are ongoing. (Troop) transfers and preparations are still underway," state news agency Anadolu quoted him as saying.

The Kurdish-led Syrian Democratic Forces took heavy losses in the US-backed campaign against the Islamic State group in Syria which they led to its successful conclusion in March.

Trump has faced a barrage of criticism, including from close allies in Washington, for appearing to leave US allies to their fate.

'Massive opposition'

Senior Republican senator Lindsey Graham addressed a tweet to the Turkish government saying, "You do NOT have a green light to enter into northern Syria.

"There is massive bipartisan opposition in Congress, which you should see as a red line you should not cross."

There has also been a chorus of international concern, including from US allies.

French President Emmanuel Macron is "very worried" about the planned Turkish operation, an aide told AFP on Wednesday.

Russian Foreign Minister Sergei Lavrov said that Iraqi Kurdish leaders he met earlier this week had voiced deep concern about the risks of the mixed signals from Trump.

"They are extremely alarmed that such a lightweight treatment of this extremely delicate subject could ignite the entire region," Lavrov said.

Since 2015, Russia has been main military backer of the Syrian government, which has seized on the policy shift from Trump to try to persuade the Kurds to accept the restoration of central government control.

The Kurds called on Moscow on Wednesday to facilitate dialogue with Damascus.

The Kurds have warned that a Turkish offensive would reverse the military gains achieved against IS and allow the jihadist group's surviving leaders to come out of hiding.

While a Kurdish-led operation earlier this year saw the death of IS's territorial caliphate, the organisation is not dead and sleeper cells have been active in several parts of Syria and Iraq.

IS claimed an overnight suicide attack by two of its fighters in its former Syria stronghold of Raqa.

Kurdish-led forces which took the city from the jihadists said the attack was an "initial repercussion" of the planned Turkish assault.