RIGA, Latvia—NATO foreign ministers are meeting here Tuesday to calibrate a response to Russia’s military buildup around Ukraine, seeking to deter the Kremlin from any potential assault while making clear the limits of its support.

Concern over the Russian buildup has risen in recent weeks. The U.S. has briefed allies that Moscow could launch a new military operation in Ukraine, following its seizing of Crimea and covert invasion of its neighbor’s east in 2014. Russia, which wants to draw the former Soviet republic back into its orbit, says it has no plans to invade and can move troops as it wishes on its sovereign territory.