President Donald Trump's decision authorizing about 200 U.S. troops to remain in northeast Syria indefinitely is a key step in creating a larger multinational observer force that can keep the peace and prevent a resurgence of the Islamic State group, administration and defense officials said Friday, as U.S. leaders press NATO allies to join.

The president also agreed to allow the Pentagon to keep about 200 troops at the al-Tanf garrison in southern Syria, where they train local forces and help root out remaining IS militants in the region.

Trump's decision endorses a plan pressed by U.S. military leaders for some time, calling for an international force of 800 to 1,500 troops that would monitor a safe zone along Syria's border with Turkey. The officials spoke on the condition of anonymity in order to provide details about a troop deployment before details are finalized.

Trump in December announced he was pulling all 2,000 U.S. troops from Syria quickly, but has gradually reversed course. He made this decision Thursday after being told European allies insisted on some U.S. forces remaining on the ground as part of the observer force.

His sudden order to pull all U.S. troops from Syria had shocked U.S. allies and angered the Kurds in Syria, who are vulnerable to attack by Turkey. It also contributed to Defense Secretary Jim Mattis's decision to resign, and drew fierce criticism in Congress. Sen. Jack Reed, a Rhode Island Democrat, called the decision a "betrayal of our Kurdish partners."

Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, has been leading discussions with allies on assembling the observer mission. He told reporters Friday he is confident they will step up and commit troops.

"I'm confident we can maintain the campaign" in Syria, Dunford said.

Asked about the decision to keep 400 U.S. troops in Syria, acting Defense Secretary Patrick Shanahan described it as "good progress." He spoke to reporters shortly before meeting with Turkish Defense Minister Hulusi Akar in the Pentagon. He said the meeting with Akar would be about "next steps."

Officials said the number of U.S. troops assigned to the safe zone could change, but that roughly 200 is an informed number. They said U.S. troops would remain in the area indefinitely to keep the U.S.'s Kurdish allies and Turkish forces from clashing, prevent forces loyal to Syrian President Bashar Assad from seizing the territory and minimize the risk of a resurgence of the Islamic State. A defense official said Turkish and Syrian opposition forces would not be allowed in the safe zone.

Turkey views Kurdish members of the Syrian Democratic Forces who have fought alongside the U.S. against the Islamic State as terrorists.

The SDF is currently involved in a standoff over the final sliver of land held by IS in eastern Syria, close to the Iraq border.

The U.S. is not seeking a United Nations mandate for the deployment and currently does not envision asking NATO to sponsor the mission, an administration official said, adding that the troops would not be "peacekeepers," a term that carries restricted rules of engagement. The current goal is to have the force stood up by the end of April.

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AP writers Deb Riechmann and Bob Burns contributed to this report.