REYHANLI, Turkey—The Turkish government closed the country's border crossings with Syria to all traffic except refugees on Wednesday, citing worsening security conditions after fighting between Syrian rebels and government forces escalated close to the frontier.

Turkey's economy minister announced the move—which will halt the passage of all commercial vehicles—in a news conference in Ankara, citing "serious concerns" for drivers' safety and noting that there had already been a 87% drop in trucks traveling to Syria this year....