Russian surveillance drones have started flying missions over Syria as Moscow sent two dozen more fighter jets to a growing air base on the Syrian coast, U.S. defense officials said Monday, in the latest sign that Russia is poised to use its air force to aid embattled President Bashar al-Assad.

U.S. officials have been struggling to respond to increasing signs that the Russian militaryis preparing to play a direct role in the Syrian conflict, which has dragged on for more than four years and created a massive refugee crisis sweeping across Europe.

Over the weekend, U.S. officials said, the number of Russian combat jets at the Syrian base south of Latakia grew from four to 28. […]