NATO ministers are expected on Wednesday to endorse efforts by the tiny Balkan state of Montenegro to join the 28-nation Western military alliance - a membership many analysts say could further strain military and diplomatic ties with Russia.
NATO foreign ministers meeting this week in Brussels are expected to clear Montenegro's membership application, with an invitation formally offered to the Podgorica government at a NATO summit in July 2016.
Russia has criticized earlier efforts at NATO expansion, which Moscow has described in Cold War terms as a threat to its security.
Croatia and Albania - two other Balkan nations - were the last to join the alliance in 2009.
Russia has historic ties with Montenegro's neighbor, Serbia, as well as other interests in the western Balkans. Russian President Vladimir Putin has repeatedly complained about what he describes as NATO encroachment on territories of former communist states of the Soviet-era Warsaw pact.
NATO offered Ukraine membership in 2008. But the Kyiv government in power at that time chose to remain in a non-aligned status - a position reversed last year to protest Moscow's support for pro-Russian rebels seeking autonomy in eastern Ukraine.
NATO has not acted on that request from Ukraine's pro-Western President Petro Poroshenko and is not expected to do so in the immediate future.