JARABLUS, Syria — As the many parties invested in Syria were considering how they might be affected by the United States withdrawing its troops, the Syrian regime made its own move Friday.

Syria amassed its troops at Manbij in northern Syria, near the Turkish border — a city Turkey has been threatening to attack for more than a year, and with increasing frequency of late. Manbij is currently occupied by US troops and the US-supported Syrian Democratic Forces (SDF). The SDF is an alliance of primarily Kurdish and Arab militias that have helped the United States fight the Islamic State (IS).

The Syrian regime reported early Friday that its troops had entered Manbij, though subsequent accounts from the United States said Syrian fighters had not yet entered the city. Other late reports said regime forces had established a barrier between the city and nearby Turkish troops.

President Donald Trump’s sudden announcement Dec. 19 that he will withdraw US soldiers from Syria has shuffled the cards and raised several questions about potential changes in the military map of control and the political balance of power in Syria following the withdrawal.