BRUSSELS — Defense Secretary Mark Esper said Friday, Oct. 25, that the United States would leave forces in Syria to protect oil fields and keep them from falling into the hands of the Islamic State.

The announcement marked an abrupt and partial reversal of President Donald Trump's speedy withdrawal from Syria, and it capped a meeting of NATO defense ministers that showcased intense anger among some alliance members toward Turkey for its decision to pour troops into Syria.

The move came a day after Trump said the United States would prevent the Islamic State from regaining control of oil fields in eastern Syria and signaled an increasing U.S. focus on Syrian energy assets.

"We are also considering how we might reposition forces in the area in order to make sure that we secure the oil fields," Esper said. "We are now taking some actions. I'm not going to get into the details."

He said the U.S. deployment "would include some mechanized forces," but he declined to comment on reports that the Pentagon's plans include the possibility of tanks or other armored vehicles and support personnel. The purpose of the troop repositioning is to "ensure that we can deny ISIS access to the oil fields," he said, using an acronym for the Islamic State.

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Esper said the overall U.S. withdrawal from Syria would continue.

The troops would remain in the eastern Syrian province of Deir al-Zour, where the United States has maintained a base, Esper said. He said the U.S. military remains in close contact with the Syrian Kurdish forces that it has long assisted.

Esper's announcement came a day after Trump said on Twitter that "we will NEVER let a reconstituted ISIS have those fields!"

"Perhaps it is time for the Kurds to start heading to the Oil Region!" he added.

The change further dilutes the practical effect of the U.S. pullout from Syria, which critics say essentially gave the Turkish invasion a green light and endangered a battlefield ally, the Kurdish-led Syrian Democratic Forces (SDF).

Pentagon officials have expressed concern that the unstable situation in Syria could allow the Islamic State to regain strength. Administration leaders have been discussing options, including tanks and associated U.S. troops, to protect oil fields that are currently under SDF control.

The shift to protecting oil fields also could raise issues in Congress.

The Pentagon is operating in Syria under the long-standing Authorization for Use of Military Force passed by lawmakers in the wake of terrorist attacks in September 2001 to target "nations, organizations, or persons" who were involved and to prevent "future acts of international terrorism."

While the Obama and Trump administrations have argued that allows action against the Islamic State, there are concerns that the Syrian regime or forces aligned with it also could want the oil fields. In February 2018, U.S. forces said they used airstrikes and artillery to kill more than 100 advancing Syrian forces in the same area after commanders determined that U.S. troops on the ground were in danger.

Esper was pressed repeatedly at a news conference in Brussels for more specifics about whether the plans included tanks and about how many troops would remain deployed around the oil fields, but he declined to offer more details.

One U.S. official, speaking before Esper's announcement, said an operation to secure oil fields would probably require about a company of U.S. troops at the outset, including a couple hundred service members, several tanks and supporting equipment.

The details would be decided based on what equipment is already in the theater, said the official, who spoke on the condition on anonymity because of the sensitivity of the situation.

The announcement capped two days of discussions at NATO headquarters in Brussels that were dominated by anger at the invasion of Syria by Turkey, a fellow NATO member. One after another, defense ministers castigated Turkish officials for a move that many said risked undermining European security by destabilizing northeastern Syria. Turkish forces invaded Syria to push back U.S.-allied Syrian Kurdish fighting groups that Ankara says pose a security threat to Turkey.

The Kurds played a leading role in pushing the Islamic State out of its territory in the region, and more recently they had been operating prison camps in which former Islamic State fighters and families were interned. The Turkish incursion has led to prison escapes and fears of a renewed terrorism threat to Europe.

Esper said Friday that Turkish Defense Minister Hulusi Akar informed him that Turkey has regained control of at least some alleged Islamic State detainees in Syria who escaped.

A spokeswoman for the Turkish mission to NATO did not immediately respond to a call for comment.

Turkey's agreement this week with Russia to jointly send in troops to the region to fill the vacuum created by the U.S. withdrawal has also caused heartburn at NATO, an alliance that spends much of its energy countering threats from the Kremlin. European countries fearful of a Russian security risk on their borders dislike that an ally, Turkey, is making security deals with Russia elsewhere in the world.

"These are disagreements on a very serious situation in northeast Syria," said NATO Secretary General Jens Stoltenberg, who has struggled to hold together an organization that runs by consensus.

NATO members have little recourse to take action against Turkey. The organization's rules do not allow members to be kicked out against their will; nor is it a venue for sanctions to be imposed. Individual members have broad power to put holds on decisions. And many NATO members still calculate that, even though Turkey can be a frustrating ally, alliance security is still improved with Turkey inside.

But many NATO policymakers fear the instability in Syria has been a gift to Russia. The upheaval in northern Syria has strengthened Russia's position in the Middle East at a time when European nations have voiced concern that a resurgent Russia could threaten their security.

"We are not very enthusiastic when we see Russians on former U.S. bases," said Lithuanian Defense Minister Raimundas Karoblis. "The alliance should not give gifts to a strategic competitor."

Esper meanwhile must navigate strong feelings of resentment within the Pentagon that the Trump administration has abandoned a battlefield ally and potentially provided an opening to the Islamic State, despite the president's depiction of events in Syria as a success. Trump this week said the region was safer than before, as he appeared to overlook a host of potential challenges to U.S. security including the expansion of Russia's military role in Syria and a possibility of renewed Turkish-Kurdish fighting.

On the other side of the domestic divisions, Esper must also keep in mind the animosity Trump has articulated toward NATO and his belief that the alliance has not provided value to the United States.

Esper "has an audience of one," said Jim Townsend, who served as a senior Pentagon official for NATO and Europe under President Barack Obama, referring to Trump.

This article was written by Michael Birnbaum and Missy Ryan, reporters for The Washington Post. Ryan reported from Washington. The Washington Post's Dan Lamothe in Washington contributed to this report.