Steve Herman at the White House, Carla Babb at the Pentagon and Nike Ching at the State Department contributed to this report.
Turkish military officials say their troops are ready to launch a long-planned incursion into Syria to take out Kurdish forces branded by Ankara as terrorists, but viewed by much of the West as key partners in the fight against the Islamic State terror group.
"Our glorious army is ready for Operation Safe Zone,'" the Turkish Armed Forces tweeted late Tuesday, promising to fight against what it described as Kurdish terrorists, as well as against IS, "with determination."
President Recep Tayyip Erdogan's communications director Fahrettin Altun wrote in a Washington Post op-ed that Turkish forces were partnering with the Free Syrian Army and would "cross the Turkish-Syrian border shortly."
Altun called on the international community to "rally behind Turkey's rebuilding and stabilization efforts," and said Turkey's only ambition in Syria is to "neutralize a long-standing threat against Turkish citizens."
The U.S.-backed Syrian Democratic Forces issued its own call for those involved in the coalition to defeat Islamic State and other nations in the world to act to avoid a humanitarian disaster in northern Syria.
"All indications, field information and military build up on the Turkish side of the border indicate that our border areas will be attacked by Turkey in cooperation with Syrian opposition tied to Turkey," the SDF said. "This attack will spill the blood of thousands of innocent civilians because our border areas are overcrowded."
Turkey placed a series of rocket launchers and artillery along the border on Tuesday. Kurdish officials charged on social media that some of those mortars already were being put to use, shelling positions used by the SDF near Sere Kaniye, near the Turkish border.
The SDF said there were no casualties and that it did not respond, calling the attack "unprovoked."
Kurdish officials said that Turkey also carried out an airstrike against alleged Kurdish positions in nearby Iraq, near a border crossing used to resupply Kurdish forces.
The rising tension, just days after a surprise U.S. announcement that it was pulling its forces back from the border area due to the upcoming Turkish operation, comes as U.S. President Donald Trump appeared to try to soothe both Turkey, a NATO ally, and the Kurds.
Taking to Twitter, Trump insisted he was not abandoning Kurdish that fought with U.S. and coalition partners against IS.
"We may be in the process of leaving Syria, but in no way have we Abandoned the Kurds, who are special people and wonderful fighters," Trump said.
"Any unforced or unnecessary fighting by Turkey will be devastating to their economy and to their very fragile currency,' he added. "We are helping the Kurds financially/weapons!"
In a separate set of tweets, the president also praised Turkey, inviting Erdogan to visit the White House next month (November 13), while calling Ankara a "big trading partner" and crediting the Turkish government with "helping me to save many lives at Idlib Province."
Yet, despite the social media diplomacy, U.S. military officials said they expect a Turkish incursion is still likely and confirmed that they have repositioned about 50 U.S. special forces members, who had been operating along the Turkey-Syria border, out of harm's way.
"Unfortunately, Turkey has chosen to act unilaterally," Chief Pentagon Spokesman Jonathan Rath Hoffman said in a statement Tuesday.
"As a result, we have moved the U.S. forces in northern Syria out of the path of [the] potential Turkish incursion to ensure their safety," he added. "We have made no changes to our force presence in Syria at this time."
U.S. backlash
Trump's decision late Sunday to move U.S. forces prompted an immediate backlash from U.S. lawmakers, some of whom called it a betrayal of the Syrian Kurds, viewed by many in Washington as the most dependable ally in the fight to destroy the IS caliphate. Other lawmakers warned the decision would only benefit Russia, Iran and Syrian President Bashar al-Assad, as well as allow IS to regain momentum.
On Tuesday, the former commander of U.S. Central Command, which overseas U.S. military operations in the region, lamented Trump's willingness to consent to a Turkish incursion.
"For me, the overall sentiment is one of disappointment," retired General Joseph Votel told an audience in Washington. "Disappointment that we are letting down our partners, perhaps adding to the humanitarian disaster in this region."
Votel also called the U.S. partnership with the mainly Kurdish Syrian Democratic Forces "a model of how we should be protecting our interests," adding the SDF has been a "capable and trustworthy partner."
Earlier Tuesday, SDF spokesman Mustafa Bali tweeted the mainly Kurdish forces has been "humbled by the enormous support by American people and politicians despite @POTUS decision to pave the way for Turkish invasion."
He also challenged Trump to see the situation on the ground for himself.
"We invite @POTUS to come see the progress #SDF & #US made in NE #Syria," he said. "You are welcome!"
With the U.S.-backed SDF spearheading ground operations, the U.S.-led international coalition liberated the last remnant of IS's self-declared caliphate this past March.
Since then, the United States has continued to provide support to the mainly-Kurdish SDF, as they engage in additional clearing operations against IS remnants and sleeper cells across northeastern Syria.
SDF reactions
SDF officials confirmed Tuesday that despite the withdrawal of U.S. special forces from the along the Syrian border with Turkey, they have continued to receive U.S. provisions for their anti-IS mission.
But other SDF officials have signaled they are willing to look elsewhere for help.
SDF Commander General Mazloum Kobani told NBC News that he has been forced to consider partnering with forces loyal to Assad to fend off the expected Turkish invasion.
"This is one of the options that we have on the table," he said.
IS fighters
In the meantime, there is considerable concern about what will become of the more than 11,000 IS fighters being held under SDF guard at more than 30 make-shift prisons across northeastern Syria.
SDF officials said some of the guards have been pulled to reinforce positions along the border to defend against the expected Turkish operation.
Former officials and analysts warn the likelihood at least some of these fighters will break out is alarming.
"Injecting hundreds or thousands of additional fighters on the battlefield who are resentful regarding their detention and resentful of the fact that they've been deprived of their caliphate is like pouring lighter food on a flickering insurgency," said Bradley Bowman, director of the Foundation for the Defense of Democracies' Center on Military and Political Power. "It's the worst possible thing we could be doing right now."
SDF officials have previously warned they may have no choice but to release the IS fighters, if forced to fend off a Turkish incursion.
There are also signs other players in the region are waiting to take advantage of any Turkish-Kurdish conflict.
"There has been a recent build-up of regime and Iranian forces in Deir Ezzor province," John Dunford with the Washington-based Institute for the Study War told VOA. "The regime is poised at least in Deir Ezzor to put pressure on the SDF."
