BEIRUT, Lebanon — A Syrian militia long backed by the United States mobilized its forces and warned of a “humanitarian catastrophe” on Wednesday as Turkey massed troops near the countries’ border for an incursion it said would begin “shortly.”
Turkey’s planned move to root out Kurdish militants in northeastern Syria, after President Trump ordered the withdrawal of American troops there, has sparked fierce debates in Washington and could open a dangerous new front in Syria’s eight-year-old war.
New violence between Turkey and the United States-backed Syrian Democratic Forces would pit two United States allies against each other in ethnically tinged battles, leaving Washington in an awkward position.
President Recep Tayyip Erdogan of Turkey has been threatening to send troops into northeastern Syria to uproot the militia, which the United States has partnered with for years 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.
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 observations 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 for unnecessary fighting by Turkey” would be “devastating” to its economy and currency, but without explaining what sort of action would cross the line.
Republican Senator Lindsey Graham of South Carolina addressed Turkey on his own Twitter account on Wednesday, 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.”