Russia said on Thursday that Belarus's refusal to host a military airbase backed by President Vladimir Putin had been an "unpleasant episode," a rare public display of disagreement between the close allies.
Russia and Belarus are members of a largely symbolic union state and are in talks to deepen their integration, but the airbase spat illustrates the limitations of their alliance as Moscow's ties with the West have plunged to post-Cold War lows.
Russia props up the Belarussian economy with cheap energy and loans, but Minsk is wary of allowing Russia too much influence, fearing that could eventually pose a threat to its sovereignty.
The Kremlin mounted a bid to set up the airbase in Belarus in 2015 and hoped it would host Su-27 fighter jets, but the former Soviet republic, which serves as a buffer between Russia and NATO's east European states, snubbed Moscow.
The plans to set up the base came as Moscow's ties with the West were rapidly fraying over Russia's annexation of Crimea, tit-for-tat sanctions and the war in Syria.
But Belarus said last year that it saw no need for a Russian airbase, that such a deployment risked exacerbating regional tensions and that the situations in Ukraine and Syria were more deserving of attention.
"This really (was) an unpleasant episode," Russian Foreign Minister Sergei Lavrov said when asked about the snub in an interview with the Kommersant business daily on Thursday.
"But content, not form is most important. And in terms of content, Belarusian President Alexander Lukashenko has said many times that ... Russia is a 100% ally," Lavrov said.
Belarus and Russia are currently holding talks to expand their integration, a process that has fuelled concern about a quiet annexation by Moscow.
Then-U.S. national security adviser John Bolton last month became the most senior U.S. official to travel to Belarus in years, a trip he said he wanted to use to warn Lukashenko of the security threat posed to Belarus by Russia.
Lukashenko has been at odds with the West over Belarus's human rights record for years, but is known for playing Russia and the West off against each other in order to extract concessions.