Speaking in a television interview Sunday morning, Defense Secretary Mark Esper said President Donald Trump had ordered a "deliberate withdrawal" of U.S. forces from northern Syria, an even more comprehensive withdrawal than had been previously ordered.

Appearing on CBS' "Face the Nation," Esper told host Margaret Brennan that after discussions with the president, and as Turkish troops continued their assault against Kurdish forces further than originally expected, the decision had been made to withdraw about 1,000 troops, rather than just the 50 who had been previously ordered to withdraw.

"I spoke with the president last night after discussions with the rest of the national security team and he directed that we begin a deliberate withdrawal of forces from northern Syria," Esper said, though he declined to provide a time frame for the withdrawal.

Esper's comments come as Turkey has begun a military operation in northern Syria against Kurdish groups that were allied with the U.S. in the fight against the Islamic State.

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Trump weighed in later Sunday on Twitter, criticizing "endless wars" and saying of the conflict, "Others may want to come in and fight for one side or the other. Let them!"

He later tweeted about potential sanctions on Turkey – "stay tuned!" he wrote, of sanctions legislation drafted with Sen. Lindsey Graham, R-S.C.

Lawmakers on both sides of the aisle have slammed Trump's decision to pull back U.S. forces as potentially enabling a resurgence of the Islamic State and abandoning a U.S. ally.

Senate Minority Leader Chuck Schumer, D-N.Y., called Trump "derelict in his duty," speaking at a press conference in New York City.

"He is making Americans much less safe, he is undoing years of work to curb ISIS. He is making America more susceptible to terrorism by his actions, he may not have intended to do that but that's just what has happened as a result of his actions."

Schumer said he and House Speaker Nancy Pelosi, D-Calif., would soon be introducing a joint resolution encouraging Trump to protect the Kurds, restrain Turkey, and prevent the resurgence of the Islamic State.

Rep. Adam Kinziger, R-Ill., who has been highly critical of Trump's Syria policy, said later on "Face the Nation" of the troop withdrawal, "it's going to be terrible and you know the secretary is doing his job and you know he has to follow orders."

Kinzinger said the decision put U.S. national security at risk and called the withdrawal "weak."

"We have left them to the wolves," he said of the Kurds.