Turkish forces and Syrian rebel allies will cross the Syrian border "shortly", a Turkish official 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 Kurdish militia allies.
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.
Ankara 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, according to a Reuters report.
President Tayyip Erdogan's communications director Fahrettin Altun said YPG fighters could either defect or Ankara would have to "stop them from disrupting" what he described as Turkey's struggle against Islamic State militants.
"The Turkish military, together with the Free Syrian Army, will cross the Turkish-Syrian border shortly," Altun wrote in a tweet and in a column published 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," he said.
Turkey's Demiroren news agency (DHA) said Syrian rebels travelled from northwest Syria to Turkey in preparation for the incursion in Syria's northeast.
They will be based in the Turkish border town of Ceylanpinar, it said, across the border from the Syrian town of Ras al-Ain, with 14,000 of them gradually joining the offensive.
The Hamza Brigade rebels from the National Army, the main rebel grouping that Turkey supports in northwest Syria, travelled in a convoy of buses accompanied by trucks loaded with ammunition, DHA said.
A large convoy of buses carrying Syrian rebel fighters and trucks loaded with equipment arrived at the Turkish border town of Akcakale early on Wednesday, a Reuters witness said.
At Akcakale, across from Syria's Tel Abyad, howitzers were deployed behind earth embankments and pointed towards Syria, a Reuters witness said on Tuesday. Multiple launch rocket systems were stationed at Suruc, some 60 km (40 miles) to the west, opposite the Syrian border town of Kobani, he said.
The prospect of a military offensive has unsettled Turkish financial markets, with the lira this week hitting its weakest level since late August. The currency firmed slightly to 5.8220 against the dollar on Wednesday morning.
Erdogan will visit the United States on Nov 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.
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, UN Secretary-General Antonio Guterres urged all parties in northeast Syria to exercise maximum restraint and protect civilians.
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.