Kyrgyzstan has signed an accession agreement to join the Russian-led Eurasia Economic Union (EEU), which comes into effect on January 1.
But, as President Vladimir Putin met in Moscow with leaders of the four other former Soviet republics forming the new alliance, the group already showed signs of divisions.
The new union, which is an expansion of the Customs Union grouping together Russia, Belarus, and Kazakhstan, also includes Armenia and Kyrgyzstan.
In signing the new accord on December 23, Kyrgyz President Almazbek Atambaev expressed hope that his country would become a full member by May 2015.
Armenian President Serzh Sarkisian had already signed an agreement to join the group in October in Minsk.
Russian President Vladimir Putin said the new union will have a combined economic output of $4.5 trillion and bring together 170 million people.
"All the participants of this integration process are already experiencing its real benefits," Putin said. "We are convinced that Armenia and Kyrgyzstan's membership in the Eurasian [Economic] Union meets the key national interests of both countries."
In addition to free trade, the union is to coordinate the members' financial systems and regulate their industrial and agricultural policies along with labor markets and transportation networks.
But Belarusian President Alyaksandr Lukashenka sharply criticized Moscow for a trade dispute with Minsk.
Moscow has banned imports of food products from the European Union as a response to Western sanctions against Russia over its activities in Ukraine.
In order to prevent Belarus from reselling EU products to Russia, Moscow halted imports of Belarus's own milk and meat, citing alleged sanitary reasons.
It also banned the transit of Belarusian food bound for Kazakhstan through its territory on suspicion that much of it ended up in Russia.
"Contrary to all international norms, we are being denied the right to the transit of goods from the territory of Belarus," Lukashenka said. "All of this has been imposed unilaterally, without any consultations."
Referring to Moscow's previous disputes with its closest partners since the dissolution of the Soviet Union, Lukashenka said Belarus will not tolerate "another Commonwealth of Independent States."
Meanwhile, Kazakh President Nursultan Nazarbaev suggested Russia's isolation from the West over the Ukraine crisis is creating tensions between Moscow and its closest partners.
"The instability of world markets, the policy of sanctions, the deterioration of trust between the leading world powers, the threat of aggravating the military and political situation -- all of this will impact the processes for building the Eurasian Economic Union," Nazarbaev said.
Also on December 23, Moscow hosted a summit of the Collective Security Treaty Organization (CSTO).
The regional security grouping comprises Russia, Armenia, Belarus, Kazakhstan, Kyrgyzstan, and Tajikistan.
Putin said the gathering was happening at a "difficult time," calling on members of the grouping to take coordinated action to enhance their security.
"Old problems are escalating and new ones are emerging," Putin said, adding that a "deformation of the global security system has been taking place."
He said "this requires the CSTO states to take coordinated and collective measures aimed at maintaining security in our countries and in our region as a whole."
Lukashenka said the situation near CSTO borders "has never been as tense and explosive as it is today."
With reporting by Reuters, AP, Interfax, Rossia 24, and TASS