President Vladimir Putin flew into Russia's far east Amur region on Tuesday for talks with Belarusian President Alexander Lukashenko expected to focus on Ukraine and Russian-Belarusian integration.
The two leaders were due to head to the Vostochny Cosmodrome to mark Russia's annual Cosmonautics Day, commemorating the first manned space flight made in 1961 by the Russian Soviet cosmonaut Yuri Gagarin.
They were also expected to inspect the spaceport and meet staff, and to give a joint news conference at around 1100 GMT.
Russia sent tens of thousands of troops into Ukraine on Feb. 24 from both Russian and Belarusian territory in what it called a "special military operation" designed to demilitarise and "denazify" its neighbour.
Ukraine has mounted fierce resistance and the West has imposed sweeping sanctions designed to force Russia to withdraw its forces from Ukraine.
Lukashenko, who has a track record of sometimes saying things that appear to jar with his closest ally's stated positions on a range of issues, has insisted that Belarus must be involved in negotiations to resolve the conflict in Ukraine and has said that Belarus had been unfairly labelled "an accomplice of the aggressor".