Last weekend's attacks on the port city of Mariupol in eastern Ukraine by pro-Russian separatists show that Moscow's objective is to increase the Ukrainian territory it controls, U.S. Ambassador to the United Nations Samantha Power said on Monday.

"The targets are fresh ones, but Russia's end goal remains the same: to seize more territory and move the line of Russian-controlled territory deeper and deeper into Ukraine," Power told an emergency meeting of the U.N. Security Council.