ANKARA/ISTANBUL (Reuters) - Turkey’s military struck the Syrian-Iraqi border to prevent Kurdish forces using the route to reinforce northeast Syria, as Ankara prepared to attack following a surprise U.S. troop pullback, Turkish officials told Reuters on Tuesday.

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 U.S. President Donald Trump widely criticized in Washington as a betrayal of America’s allies, the Kurds.

As Turkish troops finalize plans, the government’s scheme to move millions of refugees into conquered territory in northeast Syria is alarming some Western allies as much as the military operation itself.

For Turkey, which views Kurdish YPG fighters in northeast Syria as terrorists because of their ties to militants waging an insurgency inside Turkey, an influx of non-Kurdish Syrians would help it secure a buffer against its main security threat.

The U.S. move will leave Kurdish-led forces long allied to Washington vulnerable to attack by the Turkish Armed Forces (TSK).

Turkish President Tayyip Erdogan will visit the United States on Nov. 13 at Trump’s invitation, a White House spokesman said. On Monday, Erdogan said U.S. troops had started to withdraw after a phone call he had with Trump, adding that talks between Turkish and U.S. officials on the matter would continue.

On Tuesday, a U.S. official said U.S. Defense Secretary Mark Esper and Chairman of the Joint Chiefs of Staff General Mark Milley took part in the phone call, but the official did not provide further details.

“Given the likely military issues raised on the call between (Trump) and President Erdogan, Secretary of Defense Esper and Chairman Milley participated on the call,” said the official.

Earlier, the Pentagon said Esper and Milley had been consulted by Trump “over the last several days” about a possible Turkish strike in Syria.

“Unfortunately, Turkey has chosen to act unilaterally. As a result we have moved the U.S. forces in northern Syria out of the path of potential Turkish incursion to ensure their safety,” U.S. Defense Department spokesman Jonathan Hoffman said in a statement.

OVERNIGHT STRIKE

A Turkish security official said one of the main goals of the overnight strike was to cut off a transit route between Iraq and Syria often used by Kurdish armed groups “before the operation in Syria”.

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”.

Trump denied he had abandoned the Kurdish forces, the most effective U.S. 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.

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

Signaling 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 U.S. 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 Defense Ministry said.

Turkish army howitzers are positioned on the Turkish-Syrian border, near the southeastern town of Akcakale in Sanliurfa province, Turkey, October 7, 2019. REUTERS/Stringer

ROCKET SYSTEMS

Near the Turkish border town of Akcakale, across from Syria’s Tel Abyad, a Reuters witness saw howitzers placed behind earth embankments on Turkey’s side of the border, pointed toward Syria.

Some 60 km (40 miles) 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.

Trump’s warning on Turkey’s economy on Monday appeared aimed at placating critics who accused him of abandoning the Syrian Kurds. Democrats criticized his withdrawal decision, as did 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.”

The Kurdish-led forces have denounced the major U.S. policy shift as a “stab in the back”.

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

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

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 U.S. troops.

Russian President Vladimir Putin met with Russia’s security council on Tuesday. “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.

Turkey's lira TRYTOM=D3 lost 2% of its value against the dollar to hit its weakest since early September.