Turkey's military struck the Syrian-Iraqi border to prevent Kurdish forces using the route to reinforce northeast Syria, as Ankara prepared to attack there following a surprise US troop pullback, Turkish officials reported.

Key points: Turkey says it is ready to advance into northeast Syria now that the US has begun withdrawing troops

Turkey says it is ready to advance into northeast Syria now that the US has begun withdrawing troops Donald Trump has denied he abandoned the Kurdish forces

Donald Trump has denied he abandoned the Kurdish forces Turkish President Tayyip Erdogan will visit the United States on November 13 at Mr Trump's invitation

Turkey says it is ready to advance into northeast Syria now that the United States has begun withdrawing troops from the Turkey-Syria frontier in an abrupt policy shift by US President Donald Trump widely criticised in Washington as a betrayal of America's allies, the Kurds.

The US's decision will leave Kurdish-led forces — long allied to Washington — vulnerable to attack by the Turkish Armed Forces (TSK), which brands them terrorists because of their links to Kurdish militants who have waged a long insurgency in Turkey.

Giving details of the overnight strike, a security official said one of the main goals was to cut off a transit route between Iraq and Syria often used by Kurdish armed groups "before the operation in Syria".

"In this way, the group's transit to Syria and support lines, including ammunition, are shut off," the official said.

It was unclear what damage was done or whether there were casualties. Details of the strike, a joint operation by Turkey's intelligence service and the military, were hazy.

One official described them as an air strike, while the other said the site was made "unusable through various means".

Mr Trump meanwhile denied he had abandoned the Kurdish forces, the most effective US partners in fighting Islamic State in Syria.

But he praised Turkey as a trade partner, in a softening of tone hours after threatening to "totally destroy" Turkey's economy if it acted "off limits" in Syria.

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Turkish President Tayyip Erdogan will visit the United States on November 13 at Mr Trump's invitation, a White House spokesman said.

On Monday, Mr Erdogan said US troops had started to withdraw after a phone call he had with Mr Trump, adding that talks between Turkish and US officials on the matter would continue.

Amid deepening humanitarian concerns, UN Secretary-General Antonio Guterres urged all parties in northeast Syria to exercise maximum restraint and protect civilians.

Turkish officials said the overnight strike was to cut off a transit route between Iraq and Syria used by Kurdish armed groups. ( AP: Lefteris Pitarakis )

Signalling a further potential shift in the region's power balance, the Kurdish-led forces said they might start talks with the Syrian government and Russia to fill a security vacuum in the event of a full US withdrawal.

Turkey sought to underscore its determination to act.

"The TSK will never tolerate the establishment of a terror corridor on our borders. All preparations for the operation have been completed," the Turkish Defence Ministry said.

But a witness saw no sign of military activity near the Turkish border town of Akcakale, across from Syria's Tel Abyad. Howitzers were placed behind earth embankments on Turkey's side of the border, pointed towards Syria.

Some 60km to the west, multiple launch rocket systems mounted on two trucks were deployed behind earth embankments near Suruc, opposite the Syrian border town of Kobani. Artillery was also stationed in the area, and soldiers wandered around a nearby military camp.

US forces evacuated two observation posts at Tel Abyad and Ras al Ain on Monday, a US official said.

Mr Trump's warning on Turkey's economy on Monday appeared aimed at placating critics who accused him of abandoning the Syrian Kurds by pulling out US forces.

The decision drew criticism from Democrats and a rebuke from some of his fellow Republicans in Congress, including Senate Majority Leader Mitch McConnell.

His remarks, reiterated on Tuesday in slightly modified form, met an angry response in Turkey, including from opposition party politicians such as Iyi Party leader Meral Aksener.

"Threatening Turkey's economy is a diplomatic catastrophe," she told her party's lawmakers in a speech in parliament.

"The best response to this insolence is to go into the east of the Euphrates and break the terror corridor."

But on Tuesday, Trump tweeted: "So many people conveniently forget that Turkey is a big trading partner of the United States ... They have been good to deal with."

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As for the Kurds, Mr Trump said their "wonderful fighters" continued to receive US help with finance and weapons.

The Kurdish-led forces have denounced the major US policy shift as a "stab in the back".

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 41 seconds 41 s Donald Trump pledges to return US troops home from Syria

Mr Trump's decision to withdraw troops from northeast Syria has rattled allies, including France, one of Washington's main partners in the US-led coalition fighting Islamic State.

"Saying things with constancy and coherence is preferable to reacting to obvious hesitations from certain players, notably our American friends," French Prime Minister Edouard Philippe said in parliament, alluding to differences in the US administration.

Mustafa Bali of the Kurdish-led Syrian Democratic Forces (SDF) said the Turkish military build-up on the border, together with information about further mobilisation of Turkey-backed Syrian rebels, indicated that "an attack is imminent".

Syrian Kurdish official Badran Jia Kurd said Kurdish-led authorities in northern Syria may open talks with Damascus and Russia if US forces fully withdrew from the Turkish border area.

"At that time we may hold talks with Damascus or the Russian side to fill the void or block the Turkish attack," he said.

Syria insisted it would defend itself against any Turkish assault. Damascus, however, was unable to prevent Turkey taking control of a swathe of northwestern Syria earlier in the war.

Russia, Syrian President Bashar al-Assad's strongest foreign ally, said it was not told in advance by Washington or Turkey about any agreements to pull US troops from the northeast, adding it was watching the situation very closely.

Russian President Vladimir Putin and Syrian President Bashar al-Assad in Syria, 2017. ( REUTERS: Sputnik/Mikhail Klimentyev )

Russian President Vladimir Putin met with Russia's security council on Tuesday where Syria was discussed.

"Participants of the meeting have noted that, at this stage, it was important for everyone to avoid any action that could create obstacles to a peaceful settlement in Syria," the Kremlin said on its website.

Iran, another Assad ally, voiced opposition to any Turkish operation in Syria.

Germany and Britain expressed concern about Turkey's plans for military action.

Turkey, which hosts 3.6 million Syrian refugees, plans to resettle two million refugees in northern Syria. Turkey's lira lost 2 per cent of its value against the dollar to hit its weakest since early September, but edged off its lows to 5.8195 on Tuesday.

Reuters