BEIRUT, Lebanon — Turkey launched a planned military operation in northeastern Syria on Wednesday aimed at flushing out a Syrian militia backed by the United States, President Recep Tayyip Erdogan wrote on Twitter.
Mr. Erdogan said the operation aimed to “prevent the creation of a terror corridor across our southern border,” but provided no other information about whether Turkish ground troops had entered Syria or how far in they would go.
A spokesman for the United States-backed militia, the Syrian Democratic Forces, said that Turkish warplanes had begun carrying out airstrikes.
Civilians were reported to be fleeing the border towns of Ras al Ain and Tel Abyad, which were being pounded by airstrikes and shelling, Reuters reported. “There is a huge panic among people of the region,” the spokesman, Mustafa Bali, wrote.
Turkey’s long-planned move to root out United States-allied Kurdish forces in northeastern Syria accelerated rapidly after President Trump seemingly gave a green light in a call with Mr. Erdogan on Sunday. The operation has sparked fierce debates in Washington and could open a dangerous new front in Syria’s eight-year-old war.
Earlier Wednesday, the Syrian Democractic Forces had mobilized and warned of a “humanitarian catastrophe” as Turkey massed troops near the countries’ border for an incursion it said would begin “shortly.”
New violence between Turkey and the United States-backed Syrian Democratic Forces pits two United States allies against each other in ethnically tinged battles, leaving Washington in an awkward position.
Mr. Erdogan has been threatening to send troops into northeastern Syria to uproot the militia, which the United States has partnered with for years to fight the Islamic State, also known as ISIS. Turkey considers the militia a terrorist organization linked to a Kurdish guerrilla movement.
In an op-ed in The Washington Post on Wednesday, Fahrettin Altun, Turkey’s communications director, wrote that Turkish forces, with their Syrian rebel allies, “will cross the Turkish-Syrian border shortly.”
“Turkey has no ambition in northeastern Syria except to neutralize a longstanding threat against Turkish citizens and to liberate the local population from the yoke of armed thugs,” he wrote.
For its part, the Syrian Democratic Forces said the area was “on the edge of possible humanitarian catastrophe” because of the looming Turkish incursion.
“This attack will spill the blood of thousands of innocent civilians because our border areas are overcrowded,” the group said in a statement.
The Kurdish-led administration that governs the area issued a call for “general mobilization” to fight the Turks.
“We call upon our people, of all ethnic groups, to move toward areas close to the border with Turkey to carry out acts of resistance during this sensitive historical time,” it said.
Early Wednesday, Mr. Trump reiterated his opposition to United States military presence in the Middle East, writing on Twitter that “USA should never have been in Middle East.”
He said that Turkey should take control of captured Islamic State fighters from Europe whose countries had refused to take them back and who are were imprisoned in northeast Syria.
“The stupid endless wars, for us, are ending!” Mr. Trump wrote.
Tens of thousands of Islamic State fighters and their families are in prisons and camps overseen by the Syrian Democratic Forces, whose leaders say there have been no discussions with the United States about handing over the facilities.
A military coalition led by the United States partnered with a Kurdish militia in northeastern Syria beginning in 2015 to fight Islamic State extremists who had seized a territory the size of Britain that spanned the Syrian-Iraqi border.
That militia grew into the Syrian Democratic Forces and eventually took control of the areas liberated from the Islamic State, pushing it from its last foothold in Syria earlier this year.
But the partnership angered Turkey, which considers the militia a part of the Kurdistan Workers’ Party, or P.K.K., a Kurdish guerrilla movement that has been fighting the Turkish state for decades.
In recent days, Turkey has been preparing an incursion, with forces bused to the border and howitzers positioned behind dirt embankments, pointed at Syrian territory.
After a phone call with Mr. Erdogan on Sunday, the White House announced that Turkey would be sending forces into Syria and said the United States would not help or hinder their advance.
On Monday, United States soldiers withdrew from observation posts near the Syrian border towns of Tel Abyad and Ras al Ain, in the area where Turkey is expected to enter.
His forces have been key to the United States effort to defeat the Islamic State in Syria, battles that left them holding more than a quarter of Syrian territory.
Mr. Kobani and a range of current and former United States officials have warned that a new fight with Turkey could pull his forces out of areas where the Islamic State remains a threat, opening a void that could benefit President Bashar al-Assad of Syria and his Russian and Iranian backers, or the jihadists.
Mr. Trump has repeatedly sought to withdraw the roughly 1,000 American troops posted in northeastern Syria as part of his longstanding promise to extricate the United States from what he deems “endless wars.”
But he has faced fierce pushback from others in Washington, including from Republican lawmakers.
On Tuesday, Mr. Trump sought to clarify his position, writing on Twitter that the United States had “in no way abandoned the Kurds,” but that it also had good trade relations with Turkey.
He threatened that “any unforced or unnecessary fighting by Turkey” would be “devastating” to its economy and currency, but without explaining what sort of action would cross the line.
Senator Lindsey Graham, Republican of South Carolina, addressed Turkey on his own Twitter account on Tuesday, warning the country not to go ahead with the operation.
“To the Turkish Government: You do NOT have a green light to enter into northern Syria,” Mr. Graham wrote. “There is massive bipartisan opposition in Congress, which you should see as a red line you should not cross.”