ANKARA (Reuters) - Turkish forces, together with the rebel Free Syrian Army, will cross the Syrian border “shortly”, President Tayyip Erdogan’s communications director said early on Wednesday, as a surprise withdrawal of US troops made way for the Turkish incursion. Turkey has been poised to advance into northeast Syria since the US troops began vacating the area in an abrupt policy shift by US President Donald Trump widely criticised in Washington as a betrayal of America’s allies, the Kurds. On Tuesday, Turkish officials told Reuters that the military had struck the Syrian-Iraqi border to prevent Kurdish forces using the route to reinforce the region, though details of the strikes were hazy. In a tweet, President Erdogan’s aide Fahrettin Altun said that Kurdish militants there could either defect or Ankara would have to “stop them from disrupting” Turkey’s struggle against the Islamic State militants. Turkey has said it intends to create a “safe zone” in order to return millions of refugees to Syrian soil, but the scheme has alarmed some Western allies as much as the risks posed by 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. withdrawal from the area will leave Kurdish-led forces long allied to Washington vulnerable to attack by the Turkish Armed Forces (TSK). Erdogan will visit the United States on November 13 at Trump’s invitation, a White House spokesman said. On Monday, Erdogan said US troops had started to withdraw after a phone call he had with Trump, adding that talks between Turkish and US officials on the matter would continue. “Unfortunately, Turkey has chosen to act unilaterally. As a result we have moved the US forces in northern Syria out of the path of potential Turkish incursion to ensure their safety,” US Defence Department spokesman Jonathan Hoffman said in a statement. 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. 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. Amid deepening humanitarian concerns, United Nations Secretary-General Antonio Guterres urged all parties in northeast Syria to exercise maximum restraint and protect civilians. SHIFTING ALLEGIANCES Kurdish-led forces have denounced the US policy shift as a “stab in the back”. 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. Trump 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. 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 towards 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. Mustafa Bali of the Kurdish-led Syrian Democratic Forces (SDF) said the Turkish military buildup, together with information about further mobilisation of Turkey-backed Syrian rebels, indicated that “an attack is imminent”.