WASHINGTON — In a sign that the tense crisis in Ukraine could soon escalate, Russia has continued to deploy air defense systems in eastern Ukraine and has built up its forces near the border, American officials said on Wednesday.

Western officials are not sure if the military moves are preparations for a new Russian-backed offensive that would be intended to help the separatists seize additional territory.

Some analysts say that President Vladimir V. Putin may not want to act before the European Union reassesses in June whether to ease or keep the economic sanctions it has imposed on Russia because of its annexation of Crimea and intervention in eastern Ukraine. Others say it is possible that Russia is making the moves to increase the pressure on the Ukrainian government to make concessions to the separatists on political and constitutional issues.

Either way, the new military activity is a major concern because it has significantly reduced the amount of warning that Ukraine and its Western supporters would have if Russian forces and separatists mounted a joint offensive. And some of Russia’s actions, American officials say, are flagrant violations of the cease-fire agreement that European nations negotiated with Russia and Ukraine in February.