Akcakale (Turkey), Oct 10 (AP) Warning of a "humanitarian catastrophe," Syrian Kurdish forces who are allied with the United States issued a general mobilisation call as Turkey threatened to invade northeastern Syria.

The Turkish operation would ignite new fighting in Syria's 8-year-old war, potentially displacing hundreds of thousands of people, and the Britain-based Syrian Observatory for Human rights reported that people had begun fleeing the border town of Tal Abyad.

Kurdish politician Nawaf Khalil, who is in northern Syria, said some people were leaving the town for villages farther south.

Turkey has long threatened to attack the Kurdish fighters whom Ankara considers terrorists allied with a Kurdish insurgency in Turkey.

Associated Press journalists on the Turkish side of the border overlooking Tal Abyad saw Turkish forces crossing into Syria in military vehicles Wednesday, although there was no official statement from either side that the offensive had begun.

Expectations of an invasion increased after US President Donald Trump on Sunday abruptly announced that American troops would step aside ahead of the Turkish push, a shift in US policy that essentially abandoned the Syrian Kurds, longtime US allies in the fight against the Islamic State group in Syria.

But Trump also threatened to "totally destroy and obliterate" Turkey's economy if the Turkish push into Syria went too far.

Turkey has been massing troops for days along its border with Syria and vowed it would go ahead with the military operation and not bow to the US threat.

A senior Turkish official said Turkey's troops would "shortly" cross into Syria, together with allied Syrian rebel forces to battle the Kurdish fighters and also IS militants.

Trump later cast his decision to pull back US troops from parts of northeast Syria as fulfilling a campaign promise to withdraw from the "endless war" in the Middle East.

Republican critics and others said he was sacrificing an ally, the Syrian Kurdish forces, and undermining Washington's credibility.

Fahrettin Altun, the Turkish presidency's communications director, called on the international community in a Washington Post op-ed published Wednesday to rally behind Ankara, which he said would also take over the fight against the Islamic State group.

Turkey aimed to "neutralize" Syrian Kurdish militants in northeastern Syria and to "liberate the local population from the yoke of the armed thugs," Altun wrote.

Turkish President Recep Tayyip Erdogan discussed plans for the incursion with Russian President Vladimir Putin.

Erdogan's office said the Turkish leader told his Russian counterpart by phone that the planned military action in the region east of the Euphrates River "will contribute to the peace and stability" and also "pave the way for a political process" in Syria.

Turkey's Defence Minister Hulusi Akar told the state-run Anadolu Agency that Turkish preparations for the offensive were continuing.

In its call for a general mobilization, the local civilian Kurdish authority known as the Autonomous Administration of North and East Syria, also asked the international community to live up to its responsibilities as "a humanitarian catastrophe might befall our people."

"We call upon our people, of all ethnic groups, to move toward areas close to the border with Turkey to carry out acts of resistance during this sensitive historical time," it said, adding that the mobilization would last for three days.

The Kurds also said that they want the US-led coalition to set up a no-fly zone in northeastern Syria to protect the civilian population from Turkish airstrikes.

The US-backed Syrian Kurdish group urged Moscow to broker and guarantee talks with the Syrian government in Damascus in light of Turkey's planned military operation.

The Syrian Kurdish-led administration said in a statement it is responding positively to calls from Moscow encouraging the Kurds and the Syrian government to settle their difference through talks.

Syria's Foreign Ministry condemned Turkey's plans for an invasion, calling it a "blatant violation" of international law and vowing to repel the incursion.

Although it blamed some Kurdish groups for what is happening, saying they were being used as a tool to help an alleged "American project," it said Syria is ready to welcome back its "stray sons if they return to their senses," referring to the pro-US Kurdish fighters.

Russian Foreign Minister Sergey Lavrov accused Washington of playing "very dangerous games" with the Syrian Kurds, saying that the US first propped up the Syrian Kurdish "quasi state" in northeastern Syria and is now withdrawing its support.

"Such reckless attitude to this highly sensitive subject can set fire to the entire region, and we have to avoid it at any cost," he said during a visit to Kazakhstan.

Russian news media said Moscow communicated that position to Washington.

Earlier Wednesday, IS militants targeted a post of the Kurdish-led Syrian Democratic Forces in the northern Syrian city of Raqqa, which was once the de facto IS capital at the height of the militants' power in the region. (AP)

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