(CNN) The US military has moved additional troops into Syria in recent days to help provide protection to other US service members as they withdraw under President Donald Trump's directive to pullout all US troops from the country, according to two US defense officials. The officials would not reveal where in the country the troops are or how many have been sent.

Defense officials have acknowledged that a security force of armed troops -- possibly infantry -- would be needed to help carry out the withdrawal of US forces over time. The additional troops that have moved in are needed to provide security for troops and equipment as they are moved out by land and air as well a provide additional security on the ground as the number of US forces dwindles, officials said.

The officials said the additional security forces may move around Syria to different locations as needed and may move in and out of the country at times.

They would not indicate if any troop withdrawals have taken place or will happen in the immediate future, citing security concerns .

The number of US troops routinely deployed in Syria to fight ISIS, and assist local forces, has hovered between 2,000 and 2,500 in recent months at locations in northern, eastern and southern Syria. All of the current missions are continuing the official said. But with the new personnel moving in and out of Syria, and existing forces expected to withdraw, the overall number will now fluctuate.

The Pentagon is not expected to publicly discuss any new information about the number of forces on the ground in Syria due to security concerns, one of the defense officials said. Those concerns are focused on the threat posed by ISIS, Syrian regime forces and Iranian backed militias, the official said.

The news comes after ISIS claimed responsibility for a deadly explosion that killed four Americans and at least 10 other people in the Syrian city of Manbij last week.

The American deaths included two US service members, a defense contractor and a Department of Defense civilian, the US Central Command said in a statement. Three other US service members were injured in the attack.

The attack came less than a month after Trump announced that US troops would withdraw from Syria. In making his announcement, Trump declared in a video released on Twitter: "We have won against ISIS. We've beaten them and we've beaten them badly. We've taken back the land and now it's time for our troops to come back home."

Following the attack, two US officials told CNN that there are no current plans to reverse Trump's decision to begin withdrawing US troops from Syria.

The President continues to believe that it's time for US troops to return home, the officials said. Specific withdrawal plans remain contingent on events on the ground, including the strength of ISIS but also security guarantees for the Kurds.

In recent weeks senior US officials, including Secretary of State Mike Pompeo and national security adviser John Bolton, have refused to commit to a time line for the troop withdrawal.