US withdraws troops from northern Syria as Turkey expands military operation

The U.S. is withdrawing its forces from northeast Syria as Turkey's military operation targeting America's Kurdish allies there continues to expand, according to a senior U.S. official.

Defense Secretary Mark Esper said on Fox News Sunday that President Donald Trump "directed that we begin a deliberate withdrawal of U.S. forces from the northern part of Syria." But a U.S. official told ABC News the Pentagon is working to convince the president to keep a residual U.S. force elsewhere in the country.

Esper said America's partner to fight the Islamic State in Syria -- the Syrian Democratic Forces (SDF) -- is expected to "cut a deal with the Syrian and Russians" in order to gain protection from Turkey.

"Now what we're facing is U.S. forces trapped between a Syrian-Russia army moving north to take on the Turkish army that is moving south," Esper said. "It puts us in a terrible position. And the protection, safety of our service members comes first to me."

The defense secretary would not say how long it would take to move "less than 1,000" of those U.S. forces from the area, but a second U.S. official said the "planning and execution is accelerating." A source familiar with U.S. military operations in the country told ABC News that U.S. troops were destroying classified materials in preparation for their withdrawal.

"The United States doesn't have the forces on hand to stop an invasion of Turkey that is 15,000 strong if you will, proceeded by airstrikes and artillery and mechanized forces," Esper said. "You got to keep in mind to that look we didn't sign up to fight Turkey, a longstanding NATO ally on behalf of the SDF."

One week ago, the White House announced it was moving fewer than 50 American forces away from the Turkey-Syria border after a phone call between Trump and Turkish President Erdogan. That decision drew backlash from Republican and Democratic lawmakers who charged the Trump administration with abandoning America's Kurdish allies and green-lighting a Turkish invasion of Syria.

Asked what his message is to the SDF now, Esper said, "We are doing everything we can to get the Turks to stop this egregious behavior."

But in a tweet on Sunday, the president was more dismissive of the situation, writing, "Others may want to come in and fight for one side or the other. Let them!"

Meanwhile, on the ground, the violence continues to escalate with Turkish-backed militias carrying out brutal executions along the main highway. On Saturday, a 35-year old female Kurdish politician was shot in the head by what is believed to be an al-Qaeda linked group backed by Turkey.

The United Nations estimates more than 100,000 people have fled their homes. With the Turkish operation only in its fifth day, those numbers are expected to rise dramatically.

On Sunday, Turkish airstrikes hit a Kurdish convoy near the border town of Ras al-Ayn, killing at least nine people including five civilians, according to The Associated Press. The British-based Syrian Observatory for Human Rights said that journalists were among the victims.

Turkish airstrikes have also caused some SDF fighters to leave their positions guarding ISIS prison camps with reports of some ISIS fighters escaping detention. Near Ain Issa, SDF forces were forced to leave their posts at a refugee camp following a barrage of Turkish airstrikes, allowing nearly 800 ISIS women and children to flee, a senior camp official told ABC News. The Kurdish Red Crescent later confirmed that the Turkish attack caused a "large number of ISIS families" to leave.

Amid fears that a wave of ISIS families or fighters could head east, Iraq is sending more troops to its border with Syria, an official said. In a report earlier this summer, the Pentagon warned of an ISIS insurgency in Iraq and resurgence in Syria.

"ISIS will resurge. It's absolutely a given that they will come back," former defense secretary James Mattis told NBC News on Sunday.

ABC News' Ian Pannell, Martha Raddatz, Cindy Smith, James Meek, and Matt McGarry contributed to this report.