President Obama reassured allies in Eastern Europe Wednesday in a speech aimed at denouncing Russian intervention in Ukraine, as Putin unveiled a new ceasefire plan, news sources report.

Obama promised the Baltic nations of Estonia, Latvia and Lithuania, each a former Soviet republic, that NATO would defend their independence from Russia on the eve the military alliance’s summit in Wales, according to the Los Angeles Times.

At the time of Obama’s comments, Russian President Vladimir Putin announced a new ceasefire plan for the conflict thathas pitted a West-leaning government based in Ukraine’s capital, Kiev, against pro-Russian separatists in the east, Reuters reported.

Reuters reported that Ukraine Prime Minister Arseny Yatseniuk promptly dismissed Putin’s outline for peace as a “deception.”

The United States and various European governments have accused Russia of aiding the rebellion in Ukraine’s east. On Wednesday, France signaled its disapproval of Russia’s handling of the conflict, the Los Angeles Times reported, by suspending the planned delivery of a warship.

In his comments Wednesday, Obama called Russia’s alleged intervention in Ukraine a “brazen assault” on the country’s sovereignty and territorial integrity, and said the United States would never accept a Russian takeover of any tract of Ukrainian territory, the New York Times reported.

Russia annexed the breakaway Ukrainian province of Crimea in March.

“It challenges that most basic of principles of our international system — that borders cannot be redrawn at the barrel of a gun; that nations have the right to determine their own future,” Obama said as he called for military assistance to help Ukraine modernize its armed forces, the New York Times reported.

The current antagonism between the United States and Russia has rekindled memories of Cold War tensions, though Obama has said he rejects the comparisons to the long-term period of political conflict with the former Soviet Union, according to the New York Times.