MOSCOW — President Vladimir V. Putin signed a treaty on Monday to expand Russia’s authority over Abkhazia, a breakaway region of Georgia, effectively giving the Kremlin a dominant role in military and economic policy and making it easier for residents of Abkhazia to obtain Russian citizenship.

The accord comes at a time of deep apprehension in the West over Russia’s expansionist aspirations because of its annexation of Crimea and its support of the violent separatist uprising in eastern Ukraine. The treaty was angrily denounced by Georgia as illegal and a step toward “de facto annexation.”

Georgia, like Ukraine, has been moving toward closer political and economic ties with the European Union. Officials in Tbilisi, the Georgian capital, said the new accord with Abkhazia was yet another attempt by Russia to thwart the westward ambitions of a former Soviet republic while tightening the Kremlin’s influence.

“The signature of the so-called treaty constitutes a deliberate move by Russia in reaction to Georgia’s European and Euro-Atlantic aspirations,” the Georgian foreign minister, Tamar Beruchashvili, said in a statement.