U.S. President Barack Obama said Wednesday that Russia is conducting “a brazen assault on the territory of Ukraine” and called Moscow’s annexation of Crimea earlier this year unacceptable. In a joint news conference in Estonia with President Toomas Hendrik Ilves, Obama said the U.S. prefers a “strong, productive, cooperative Russia,” but that can’t be achieved unless Moscow ends its subversion and aggression in Ukraine.

“Borders cannot be redrawn at the barrel of a gun,” Obama said in a speech in Tallinn, the capital of Estonia. He said Russia has destabilized Ukraine by lending military support to pro-Russia separatists, who he said in some cases “are not homegrown.”

“We must continue to stand united against Russia’s aggression in Ukraine,” Obama said, referring to the Baltic states of Estonia, Latvia and Lithuania, former Soviet republics that are now NATO members. He added that the coming NATO summit in Wales will be used to reiterate the alliance’s political support of Ukrainian President Petro Poroshenko. Ukraine is not a member of NATO, although Russia’s involvement in the eastern portion of the country is swaying Kiev’s leaders to pursue membership.

Obama said Russia needs to abide by international norms and improve its economy. “That’s not the path that they’ve been pursuing over the last several years. It’s certainly not in evidence when it comes to their strategy in Ukraine,” he said.