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 U.S. troops made way for the Turkish incursion.
Turkey has been poised to advance into northeast Syria since the U.S. troops began vacating the area in an abrupt policy shift by U.S. President Donald Trump widely criticised in Washington as a betrayal of America's allies, the Kurds.
Turkish officials earlier said the military had struck the Syrian-Iraqi border to prevent Kurdish forces using the route to reinforce the region.
Erdogan's aide Fahrettin Altun said that fighters from the Kurdish YPG militia in Syria could either defect or Ankara would have to "stop them from disrupting" Turkey's struggle against the Islamic State militants.
"The Turkish military, together with the Free Syrian Army, will cross the Turkish-Syrian border shortly," Altun said in a column in the Washington Post.
"Turkey has no ambition in northeastern Syria except to neutralize a long-standing threat against Turkish citizens and to liberate the local population from the yoke of armed thugs," the column said.
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 Nov. 13 at Trump's invitation, a White House spokesman said. He announced on Monday that U.S. troops had started to withdraw after a phone call he had with Trump.
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.
Amid deepening humanitarian concerns, United Nations Secretary-General Antonio Guterres urged all parties in northeast Syria to exercise maximum restraint and protect civilians.
Kurdish-led forces have denounced the U.S. 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 U.S. withdrawal.
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.
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".