Russia and Belarus have begun their biennial joint military exercises involving some 200,000 troops and support staff, 300 tanks, 80 aircraft and helicopters, and 15 ships, the Russian Defence Ministry announced on Friday.
The war games, which are given the name Zapad ('West'), will last until September 16 and occur at a time of significant tension between both countries and their European neighbours.
NATO Secretary General Jens Stoltenberg called on Russia to be transparent about the number of participants involved, noting that in the past more troops have been mobilized than announced, "so we will be vigilant," he said.
NATO has criticized Russia for not inviting western observers to the joint exercises even once since the end of the Cold War, a requirement under international agreements to which both countries are signatories.
When more than 13,000 troops are participating in military exercises, allowing observers was mandatory, NATO said.
Meeting in Moscow
During a meeting with Belarusian President Alexander Lukashenko in the Kremlin on Thursday evening, Russian President Vladimir Putin was keen to emphasize that the manoeuvres were not directed against anyone in particular.
"However, conducting these exercises is logical, given that other alliances such as NATO, for example, are moving fast to build their military presence close to the borders of the Union State," Putin said, referring to the economic union between Russia and Belarus that has existed since 1999, and which Putin appears keen to elevate to a full political union, something the two men discussed during their meeting.
The military exercises served as a signal to the West that they cannot talk down to Minsk or Moscow, according to Viktor Gulevich, the Belarus military chief of staff, speaking at the beginning of the exercises.