Abkhazian and South Ossetian lawmakers will now be able to address parliamentary assembly sessions of the union state, Nikolai Tcherginetz, who heads the assembly's commission on foreign affairs said.
Russian media quoted a deputy speaker of South Ossetia's legislature, Yury Dzitstsoity, as saying that if Belarus recognized the breakaway republics, they would become permanent members of the assembly.
Belarusian President Alexander Lukashenko earlier said his country would consider the regions' recognition request. Russia recognized the regions as independent states after a brief war with Georgia, which attacked South Ossetia in an attempt to regain control in early August.
Nicaragua has so far been the only other country that has followed Russia in recognizing both breakaway states.
The Caucasus conflict is the focus of international talks in Geneva, the next round of which will take place on November 18. The talks were suspended on Wednesday after Russia insisted Abkhazia and South Ossetia should participate and Georgia objected.
Abkhazia and South Ossetia have been de facto independent republics since they broke away from Georgia after the bloody post-Soviet conflicts in the early 1990s.
The Russia-Belarus Union State is a supranational entity consisting of the Russian Federation and the Republic of Belarus. It was formed in 1996 "with the intention of providing greater political, economic, and social integration."