Russian parliament's upper house on Tuesday voted to allow President Vladimir Putin to use the Russian army outside the country to support separatists in Ukraine.
A total of 153 Russian senators backed the decision, with no one voting against or abstaining.
Putin earlier in the day asked the Federation Council to approve the army's use outside the country to back separatists who have been fighting the Ukrainian army since 2014.
"Negotiations have stalled. The Ukrainian leadership has taken the path of violence and bloodshed," Deputy Defense Minister Nikolay Pankov said during a session of the Federation Council called at Putin's request.
"They have not left us a choice," Pankov said, addressing the chamber.
He alleged that there were "heavy armored vehicles" on the border of east Ukraine's separatist-controlled regions known as the DNR and LNR.
Pankov also said that NATO was "actively pumping Ukraine with modern weapons."
"Russia will act for the protection of the sovereignty of other states and in order to prevent acts of aggression," Pankov said.
He cited Putin's request: "In accordance with the treaty of friendship and cooperation with the DNR and LNR, I am submitting a proposal for the adoption by the Federation Council for the consent to use the armed forces of the Russian Federation outside Russia."
On Monday, Putin recognized the independence of the Donetsk and Lugansk regions and signed agreements with them, opening the door for the presence of the Russian army in Ukraine's rebel territories.