Ivanov said during a visit to the Belarusian capital Minsk that the expansion requires closer cooperation between the two countries in "both the military-political, as well as purely military spheres," Interfax reported.
"The military potential of our two countries is able to stabilize the international situation," Ivanov said in televised remarks.
He did not go into further details.
Russia and Belarus have been mulling a union for years, also discussing military cooperation, but the union never materialized amid Moscow concerns about the unpredictable Belarus president, Alexander Lukashenko.
Russia has opposed NATO's expansion, particularly into the three Baltic states.