US President Barack Obama saluted the "milestone" end of NATO's combat mission in Afghanistan, but warned the country remains "a dangerous place."
NATO will hold a ceremony in Kabul later formally ending its war in Afghanistan, officials said, after 13 years of conflict.
The Kremlin branded the expansion of NATO as a fundamental threat to Russia in a revised military doctrine that dramatically reflects deteriorating relations with the West.
President Barack Obama in a Christmas Day speech to US troops stationed in Hawaii hailed their "extraordinary dedication and sacrifices."
Ukraine took a historic step toward NATO in a parliamentary vote that stoked Russia's anger ahead of talks on ending the ex-Soviet state's separatist war.
Ukraine's president openly challenged the Kremlin by submitting a bill allowing the former Soviet republic to join NATO and make the Western alliance its defender.
Recent close shaves between Russian fighters and civilian aircraft highlight the dangers of the cat-and-mouse game being played out between Moscow and the West.
NATO foreign ministers approved fresh support for Ukraine and condemned Russia for backing pro-Moscow rebels.
Russia lashed out at NATO, saying its military drills in the Baltics could destroy northern Europe's stability.
French authorities and NATO confirmed the presence of four Russian warships in the English Channel.
NATO's top military commander said he was "very concerned" that Russia's military build-up in the annexed Crimean region could be used as a launchpad for attacks.
Ukraine's pro-Western political parties agreed to make joining NATO a priority in a coalition deal that will give them a parliamentary majority.
NATO accused Russia of sending tanks, troops to east Ukraine, raising fears of renewed all-out fighting and calling into question the Minsk peace deal.
NATO member Poland invited manufacturers of short-range surface-to-air missiles for technical talks for a planned military revamp amid heightened tensions with Russia.
Russia's recent military flights into European airspace are meant to demonstrate to the West that the country is a "great power".
NATO reported more Russian military flights and interceptsover the Baltics, North Sea and Atlantic, just days after it picked up "an unusual level" of activity over European airspace.
NATO said it had tracked and intercepted four groups of Russian warplanes "conducting significant military manoeuvres" in European airspace over the past two days.
Disassembled airplanes of unclear origin have been delivered to the Aktau airport in Western Kazakhstan.
Cuba's former president Fidel Castro on Wednesday accused NATO of planning a "war of extermination" against Russia.
NATO's top military commanders are meeting in Lithuania to discuss relations with Russia.