President Trump announced on Monday that U.S. troops in northeastern Syria will be withdrawn from the country as planned, and redeployed “in the region to monitor the situation and prevent a repeat of 2014,” when the Islamic State made major territorial gains.

His statement came as two U.S. officials told Fox News that the Pentagon was accelerating its plans to get most U.S. troops out of Syria in the coming days and weeks. A “small footprint” of American forces will remain at a garrison in southern Syria to continue to disrupt remnants of ISIS, the president said in his statement.

It's not clear right now when the roughly 1,000 U.S. troops in Syria will be pulling back. The “small footprint” the president referenced was the At Tanf garrison in southern Syria near Jordan, where troops are expected to remain to guard a supply line used by Iranian-backed forces to move weapons between Tehran and Beirut, as well as Damascus, the officials added. ISIS traditionally has not had a large presence in that part of Syria.

Defense Secretary Mark Esper released a statement a short time after Trump’s tweet saying the decision by Turkish President Recep Tayyip Erdogan to invade northern Syria has “resulted in widespread casualties, refugees, destruction, insecurity, and a growing threat to U.S. military forces.”

Turkey’s invasion has “resulted in the release of many dangerous ISIS detainees,” Esper added in the statement.

“Turkey's unilateral action was unnecessary and impulsive. President Erdogan bears full responsibility for its consequences, to include a potential ISIS resurgence, possible war crimes, and a growing humanitarian crisis. The bilateral relationship between our two countries has also been damaged,” Esper said.

The Pentagon statement said Esper will visit NATO headquarters next week in Brussels to “press our other NATO allies to take collective and individual diplomatic and economic measures in response to these egregious Turkish actions.”

The Pentagon also revealed that Chairman of the Joint Chiefs of Staff Gen. Mark A. Milley spoke with his Russian counterpart, Gen. Valery Gerasimov, over the phone Monday, without revealing specifics from their conversation.

Esper told Chris Wallace on “Fox News Sunday” that Trump had ordered a “deliberate withdrawal” from northern Syria. It was not clear if he meant leaving the country altogether.

Esper hinted the retreat from Syria could take longer because of the large number of U.S. armored vehicles and heavy weapons currently on the ground in Syria—military hardware he does not want to see fall into enemy hands.

“We want to make sure we don't leave equipment behind. So I'm not prepared to put a timeline on it,” he said on CBS News’ “Face the Nation.”

In further evidence the relationship with NATO ally Turkey was deteriorating, Fox News is told the Pentagon canceled a planned Open Skies reconnaissance flight over Russia this week with the Turkish military. The flight aboard a Turkish plane with U.S. military observers on board is part of a long-standing arms control agreement between NATO and Russia.

The Pentagon has not immediately returned a request for comment on the canceled flight.

Critics said the Trump administration has betrayed the Syrian Kurds--the main U.S. ally against ISIS.

KURDISH FORCES ANNOUNCE DEAL WITH DAMASCUS AFTER U.S. PULLOUT

“What we're seeing on the ground is absolutely sickening. It’s absolutely shameful that President Trump allowed Turkey to begin killing the Syrian Kurds who were our allies in the fight against ISIS,” Maryland Democratic Sen. Chris Van Hollen said on “Fox News Sunday.”

Esper defended the decision to retreat. On “Fox News Sunday,” Trump’s defense secretary said there was nothing the U.S. could do to stop Turkey’s invasion.

“They were fully committed to doing this, regardless of what we did. We thought it was prudent. It was my recommendation. I know the chairman of the Joint Chiefs of Staff agreed as well. We should not put U.S. forces in between a Turkish advance,” Esper said. “We're talking less than 50, more like two dozen. There is no way they could stop 15,000 Turks from proceeding south.”

Asked if Turkey had behaved like a NATO ally, Esper was candid, “The arch of their behavior over the past several years has been terrible. I mean, they are spinning out of the western orbit, if you will,” Esper said.

The situation in Syria following Turkey’s invasion was getting “worse by the hour,” he added.

Esper said Trump ordered roughly two dozen U.S. troops to pull back from Syria’s border with Turkey border to avoid being “trapped” by Russian and Iranian-backed Syrian forces pushing north and Turkish forces moving south.

Meanwhile, U.S. officials cited Turkish “propaganda” as pushing the narrative that hundreds of ISIS fighters have escaped or were let go from Kurdish-run prisons in Syria.

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The officials told Fox News that Turkish artillery units “intentionally” were shelling Kurdish-run prisons in Syria allowing ISIS fighters to escape.

Turkey claimed 800 ISIS fighters escaped the border town of Tal Abyad, but the local prison held only a “few dozen” ISIS prisoners, a U.S. official told Fox News, calling the claim “Turkish propaganda.”

Fox News' Mike Arroyo and Chris Wallace contributed to this report.