President Trump "went off script" during his call on Sunday with Turkish President Recep Tayyip Erdogan, before he announced that the U.S. would withdraw all troops from northeast Syria ahead of a Turkish military operation in the region, a well-placed senior U.S. military source told Fox News on Wednesday.

During the phone call, Trump had talking points, according to the source: “Tell Erdogan to stay north of the border."

“He went off script," the source said.

Erdogan launched Operation Peace Spring on Wednesday morning after President Trump said the U.S. would pull its troops from the area, a move many analysts and political observers have called a blow to the U.S.-backed Kurdish forces there.

Erdogan said the mission was to "neutralize terror threats" and establish a "safe zone."

At least seven civilians have been killed in Turkish strikes in northeastern Syria since Turkey began its assault, according to activists and a war monitor. Earlier Wednesday, Turkish forces carried out airstrikes in "civilian areas" and caused a "huge panic among people of the region," according to a spokesperson for the Kurdish-led Syrian Democratic Forces (SDF).

TURKEY SAYS GROUND FORCES HAVE INVADED NORTHERN SYRIA; MULTIPLE CIVILIANS REPORTED DEAD

U.S. military officials told Fox News the president ordered the military not to get involved in the strikes, after the Kurds requested air support.

Turkey announced later on Wednesday that its ground forces had invaded northeastern Syria to fight the Kurds.

Kurdish forces in northeastern Syria were guarding thousands of captured Islamic State (ISIS) fighters with the help of the U.S. in the area.

Trump told reporters Wednesday afternoon that the captured terrorists were "really bad people who should go back to Europe."

"We said to various countries, we'd like you to take your people back. Nobody wants them, they're bad," Trump said, adding that "maybe the Kurds [...] if not them, Turkey" would deal with the ISIS fighters.

According to the well-placed military source, “The president has said a number of times: 'Just let the Turks do it!’ ” in reference to Syria and fighting ISIS.

TRUMP DECISION TO PULL BACK US TROOPS IN SYRIA RECALLS PAST CONFLICTS WITH BOLTON, MATTIS

The president’s views about wanting to leave Syria have been well known since December 2018, when former Defense Secretary Jim Mattis resigned amid Trump's plans to pull out of Syria and abandon the Kurds, longtime U.S. allies in the fight against ISIS in Syria.

Senior military commanders were surprised by the timing of the president’s announcement on Sunday night.

“This is a big deal,” according to the source. “The SDF will have to reorient to the north” and stop fighting ISIS.

TRUMP PULLS BACK TROOPS FROM NORTHERN SYRIA AHEAD OF TURKISH ASSAULT, PENTAGON OFFICIALS 'BLINDSIDED'

Trump "doesn't buy the argument that ISIS unchecked will follow us home," the source said they were told from members of the president's national security team.

If the ISIS fighters escape, they'll "go back to Europe," Trump said Wednesday.

What is likely to follow will look like “ethnic cleansing” by the Turks, according to the senior military source.

“I don’t know how many people will die. A lot of people will die,” the source said, adding they were “sick about it."

There is "no plan" for the 10,000 ISIS detainees and 2,000 foreign fighters with the Kurds, all of whom have been guarded by the SDF, according to the source.

"There’s not a plan for any of that. The Turks are not" going to take responsibility for the ISIS detainees, the source said. "The Turks don’t see ISIS the same way we do."

“Strategically, everything we warned about is happening,” the military source said, referring to ISIS attacking Raqqa, Syria, and the SDF preparing to leave its positions to fight Turkey.

From a military perspective, an investment of 1,000 U.S. troops to keep stability in northern Syria was reasonable, the source maintained. “I can’t imagine where the anti-ISIS mission remains viable.”

While no decision has yet been made, the source said there are concerns about force protection and the inability of U.S. troops to continue their missions of partnering with the SDF to go after ISIS, meaning it could be all but certain the remaining 1,000 U.S. troops will be withdrawn.

“[I] can’t see why you would leave U.S. forces there" now, according to the source, who also said no one would partner with the United States in the future.

"How do you get other partners? What would give them a reason to trust us?” the source said.

Earlier Wednesday, Trump warned Turkey that its initial assault was a "bad idea."

TRUMP CALLS TURKEY ASSAULT ON SYRIA A 'BAD IDEA' AS KURDS REPORT CIVILIAN DEATHS

"The United States does not endorse this attack and has made it clear to Turkey that this operation is a bad idea," Trump said in a White House statement. "Turkey has committed to protecting civilians, protecting religious minorities, including Christians, and ensuring no humanitarian crisis takes place -- and we will hold them to this commitment."

Trump later told reporters that he would employ "far more than sanctions" against Turkey if the country fails to act in the most "humane way" possible, and that he hoped Erdogan would act rationally.

When asked what would happen if Erdogan wiped out the Kurds, Trump threatened to "wipe out" Turkey's economy, saying he'd done it once before.

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