(CNN) President Donald Trump has agreed to keep about 400 US troops in Syria after the withdrawal of most US forces there this spring, a senior administration official said Friday.

Trump agreed on Thursday to keep about 200 US troops as part of planned multinational force that would maintain a safe zone in northeastern Syria, the official said. This force would be in addition to the 200 troops the US is planning to maintain after the withdrawal at its base in at-Tanf, Syria, the official said.

Trump on Friday denied that the decision represented a reversal of his previous position.

"I'm not reversing course. I have done something nobody else has been able to do," Trump told reporters in the Oval Office.

He continued, "At the same time, we can leave a small force, along with others in the force, whether it's NATO troops or whoever it might be, so that it doesn't start up again. And I'm OK. It's a very small, tiny fraction of the people we have and a lot of people like that idea and I'm open to ideas."

Acting Secretary of Defense Pat Shanahan refused to confirm Syria troop numbers Friday while addressing reporters at the Pentagon prior to a meeting with his Turkish counterpart.

"I'm not going to talk troop numbers or troop movements but I think we saw some good progress yesterday and today's meeting really is about talking next steps," Shanahan said.

The US and NATO allies are in the process of assembling what officials are calling a "monitoring and observer" force of roughly 800 to 1,500 troops to maintain a safe zone in northeastern Syria. The goal of the force would be to maintain a buffer between Turkey and US-allied Syrian opposition forces to ensure that neither side carries out attacks on the other.

A US defense official said the force would also have the aim of preventing an ISIS resurgence in northeast Syria though it is not clear whether they would be involved in training or advising local forces as part of this effort.

The US initially planned to only provide air support -- not ground troops -- to the observer force, but NATO allies objected and said they would not contribute troops toward such a mission without a US troop commitment, the official said.

It became clear to US officials on Monday that the observer force would never become a reality absent an American commitment of ground troops. After consulting with State and Defense Department officials, national security adviser John Bolton met with Trump on Thursday and convinced him to agree to commit "a couple hundred" troops, said a senior administration official, who briefed a small group of reporters on Friday on the plans on condition of anonymity.

The defense official confirmed that Bolton convinced Trump to extend a US presence on Thursday.

A US official told CNN Thursday that the remaining US troops will be able to provide unique high-end capabilities -- such as logistics, intelligence, surveillance, reconnaissance and calling in airstrikes -- that would help encourage coalition countries like France and the United Kingdom to also keep their troops in Syria to help ensure the safe zone.

The decision ticks up the number of troops that Trump is agreeing to maintain in Syria after he hastily announced his plans to withdraw US troops from Syria late last year, stunning US officials and allies and triggering the resignation of Secretary of Defense Jim Mattis. US officials and allies have sought to mitigate the risk to US-allied Syrian opposition forces and to ensure that the US can adequately respond to the potential resurgence of ISIS in the region.

Planning for the multinational observer force stems from an idea that Turkey proposed to Bolton and the chairman of the Joint Chiefs of Staff Gen. Joseph Dunford during a visit in January, amid concerns about the potential for conflict between Turkey and US-backed Kurdish forces. The Joint Chiefs has since been leading discussions with NATO allies to assemble the force.

Turkish officials have expressed concerns that US-backed Kurdish forces would carry out terrorist attacks in Turkey, and the US-backed forces have worried about the fret of a Turkish invasion.

Speaking prior to his meeting with Dunford and Shanahan Friday, Turkish Minister of Defense Hulusi Akar told reporters that they were "here to discuss policy issue in detail," saying that Turkey had no issue with the Syrian Kurds but that they did consider Kurdish militia groups backed by the US to be linked to groups that Ankara views as terrorist organizations.

The senior administration official made clear that the observer force would not be a United Nations-chartered peacekeeping force and would not face the same restrictive rules of engagement that those forces face, but declined to elaborate on what those rules of engagement would be. The forces would, however, be combat-ready, the official said.

The Joint Chiefs of Staff are continuing to hammer out discussions with NATO allies, but given that a commitment of US troops was the Europeans' main objection, US officials are optimistic that this force will be assembled.

The senior administration official briefed reporters on Friday after White House press secretary Sarah Sanders issued a brief one-line statement asserting that "a small peace keeping group of about 200 will remain in Syria for a period of time." That figure did not take into account the presence of another 200 troops expected to remain at at-Tanf.

The defense official would not say how many troops are to remain in at-Tanf, but confirmed that the US military would maintain a presence there as part of its effort to target ISIS remnants and prevent the terror group's resurgence.

US officials hope the multinational force in northeastern Syria will minimize the risk of conflict between Turkey and US-backed forces in Syria.

US officials are aiming to withdraw most US troops from Syria by April, but a senior administration official said Friday that date could shift depending on how quickly the multinational observer force is assembled.

This story has been updated with additional developments Friday.