A Turkish plan to move refugees into zones previously controlled by U.S.-backed Syrian Kurdish forces may compound instability in already tense areas of northern Syria, Defense Secretary Mark T. Esper said Wednesday.

Military forces are seeking to navigate a complicated environment in Syria, Esper told lawmakers, following a Turkish-led offensive against Kurdish troops who have partnered with the Pentagon against the Islamic State, a move that coincided with President Trump’s decision to reduce the U.S. military footprint there.

Turkish President Recep Tayyip Erdogan on Monday said he intended to move 1 million Syrian refugees living in Turkey into northern Syria, part of a plan to ease his country’s burden and move Syrian Kurdish forces, which Ankara views as a terrorist threat, away from the Turkish border.

Speaking to the House Armed Services Committee alongside Gen. Mark A. Milley, chairman of the Joint Chiefs of Staff, Esper said the situation in northern Syria had stabilized in recent weeks in areas around a “buffer zone” that Turkey established south of its border. Both Kurdish and U.S. troops were forced to pull back from those areas as Turkish-backed forces moved in.

“The other thing we have to watch out for in the coming months is as Turkey begins to resettle the internally displaced persons . . . what’s that going to cause in terms of disruptions with the Kurds as they move them back into Kurdish areas” of Syria, Esper said. “There will be some turmoil as that happens. That’s beginning to happen now, and I think we’re going to watch that very carefully.”

Erdogan’s campaign upended a fragile balance in northern Syria enabled by a four-year U.S. military presence. It has allowed Syrian government and Russian forces to press into areas they had been locked out of for years, potentially giving Syrian President Bashar al-Assad and his allies, Iran and Russia, an upper hand in political wrangling over Syria’s future.

After Trump backed off what appeared to be a plan earlier this fall to execute a full withdrawal of U.S. troops, a reduced force was repositioned in areas of Syria away from the border. Military officials say they have resumed operations with their Kurdish-led partner force, the Syrian Democratic Forces, against the Islamic State despite the Kurds voicing a feeling of betrayal.

Erdogan’s offensive and Trump’s decision to reduce the U.S. presence have generated concerns among military officials about a possible extremist resurgence and a chilling effect on future U.S. attempts to work with local forces.

Officials say they have not detected a significant Islamic State comeback so far but expect the group to continue launching attacks and, eventually, attempt to retake territory.

Milley cautioned that a full withdrawal from Syria could allow the extremists to regroup within a year.