On Wednesday (2 December) the Balkan state of Montenegro will be formally invited to join the NATO military alliance, diplomatic sources said, a move which could further strain already difficult ties with Moscow.
The offer is expected to come after a meeting of foreign ministers from the 28-nation alliance in Brussels on Tuesday (1 December) and Wednesday.
"The proposed text has been approved at (NATO) ambassador level," one source said Monday, asking not to be named. "After that, it would take at most a year and a half for Montenegro to become a member state," the source added.
NATO chief Jens Stoltenberg said he could not confirm the decision because it was up to the grouping's foreign ministers but he commented positively on the prospect.
"Montenegro has come a long way on its path to join the Euro-Atlantic family," he told a briefing ahead of the meeting.
>>Read: Montenegro optimistic on NATO membership invite
"Extending an invitation to Montenegro to start accession talks would be a historic decision. It would signal our continued commitment to the Western Balkans," he said.
The foreign ministers' meeting is expected to be dominated by the Syrian conflict, closely followed by relations with Russia and the Ukraine crisis.
Moscow has historic ties with Montenegro's neighbour Serbia and interests in the Western Balkans, while finding itself at loggerheads with NATO over a series of issues.
Russian President Vladimir Putin bitterly complains of what he sees as NATO encroachment, especially after the pro-Western Kyiv government said it was looking to join the US-led alliance in the future.
NATO offered Ukraine membership in 2008, when Russia went to war against another former Soviet state, Georgia, but Kyiv opted for what it said was a "non-bloc" policy instead.
>>Read: EU broadens accession talks with Montenegro
President Petro Poroshenko however reversed that position last year over Moscow's support for pro-Russian rebels in eastern Ukraine and its annexation of Crimea.
Most of the former communist states of the Soviet-era Warsaw Pact have joined NATO, starting in 1999.
Balkan states Croatia and Albania were the most recent countries to join, in 2009.
After the fall of the Berlin wall, for the new members of the Western clubs, usually NATO membership has preceded EU accession. Montenegro has applied to join the EU in 2009 and received the status of candidate country in December 2010. Accession negotiations are ongoing.