JERUSALEM—Israel joined the U.S. in expressing concerns about Russia’s military buildup in Syria ahead of a planned meeting in Moscow on Monday between Prime Minister Benjamin Netanyahu and Russian President Vladimir Putin.

“Once we understand exactly what Russia intends to do in the arena and exactly what it plans to move into it, whether infantry or air forces, or others…we’ll know what needs to be done,” Yossi Cohen, Mr. Netanyahu’s national security adviser, told Israel Radio on Friday.

He...