Putin, speaking to a youth forum Russia's Monday, indicated he held out hope that Belarus, South Ossetia and Russia could join a common state.
"It is possible, very desirable and fully depends … on the Belarusian people's will," ITAR-Tass reported Putin as saying.
The prime minister added that Belarusian President Alexander Lukashenko deserved praise for "consistently moving along the path toward integration with Russia."
While Belarus and Russia in the late 1990s signed a treaty calling for the creation of a "Union State of Belarus and Russia" -- an entity that was to have a common currency -- the idea stalled and the two countries remain separate.
The Belarusian Ministry of Foreign Affairs declined to comment on Putin's statements Tuesday.
"The foreign ministry does not consider it necessary to comment on the statement," ministry spokesman Andrey Savinykh told the BelaPAN news agency. "I just would like to note that the Russian prime minister underlined that the matter depends 100 percent on the will of the Belarusian people."
Lukashenko told reporters in June that Moscow had voiced hopes Belarus could be incorporated into Russia and that Minsk should agree to the absorption if it wants to be pay the same amount for Russian natural gas as the nearby province of Smolensk.
But the Belarusian strongman vowed his country wouldn't join Russia or the European Union while he remained in power, asserting that a majority of the Belarusian people don't want to be absorbed by Russia or join the EU "on conditions similar to those set once for the Baltic states."
"I don't want this as well," BelaPAN reported him as saying. "I want us to be happy here. Such a policy and independence cost much. If you are not ready to pay, speak out, you 90 percent of the population, and you will be able to become part of another state, part of the European Union -- but without me."
In his remarks this week, Putin also expressed a hope for Russian unification with South Ossetia, one of two breakaway regions of Georgia that had proclaimed independence from Tbilisi in 2008, sparking a brief armed conflict.
Putin said Russia helped South Ossetia when it became a target of "military, provocative, criminal actions" of the Georgian leadership, adding, "the future will depend on the South Ossetian people themselves."
While unification with Russia wasn't on the agenda, Tskhinvali was "ready to enter either into union state of Russia and Belarus, or directly integrate within the Russian Federation," South Ossetian Speaker of Parliament Stanislav Kochiev said Tuesday, according to the online news site Civil Georgia.
South Ossetia's de facto ambassador to Moscow Dmitry Medoev, however, had a different take. He said Tuesday the region wasn't ready to become part of the Russian Federation and no plans for a referendum to do so are on the table, Radio Ekho Moskvy reported.
Tskhinvali "will not waste its freedom to please any political force," he said, adding, "We have many problems to solve -- the rehabilitation after the August bombing of Tskhinvali, social and economic issues. The question of a referendum is not on top of the agenda now."