The regional military group comprised of Russia and Belarus has kicked off a mission aimed at protecting the Union State of both countries, Belarusian Defense Minister Viktor Khrenin said Monday.

The first Russian air force troops arrived in Belarus on Sunday following the first convoy of Russian troops that arrived on October 15 as part of the joint regional military group the two countries agreed on earlier.

Russia and Belarus are members of the Union State, a supranational organization formed in the 1990s with the objective of integrating both nations' political, economic, and defense policies. Its military doctrine declares that any aggression against either member is aggression against both, which would call for "appropriate measures" to be taken "using all the forces and means" at the countries' disposal to neutralize the threat.

According to the Belarusian Defense Ministry, "The air force part of the regional grouping of troops started arriving in the Republic of Belarus from the Russian Federation."

A senior official in the Belarusian Defense Ministry, Valery Revenko, said Sunday on Twitter that slightly less than 9,000 Russian soldiers, would be arriving in Belarus as a member of the combined regional military force, and that the redeployment would take several days.

"The regional force grouping is a complicated system with many components. It includes command and control bodies of various levels, army units, and forces, as well as various Belarusian and Russian support units," Khrenin was quoted by Belta News Agency as saying. 

On October 10, Russian President Vladimir Putin and his Belarusian counterpart Alexander Lukashenko agreed on the deployment of a joint regional group of forces in light of recent developments in Ukraine.

Meanwhile, Belarusian Security Council Secretary of State Alexander Volfovich said that the joint regional military group is being deployed gradually and it remains to be seen if there will be need for a full-scale deployment.

"A lot of online reports say that Russian army units are being relocated to Belarus in order to create a force that will attack Ukraine. It is something our foreign neighbors would like to see, regretfully. Particularly Poles. Yes, the regional force grouping has started concentrating in Belarus' territory. Gradually and not up to full strength," Volfovich said, as quoted by Belta.

The Belarusian armed forces form the basis of the joint regional military grouping without any mobilization measures, the official underlined.

Lukashenko, at the same time, accused the NATO chief of pushing Kiev to attack Minsk. He also noted his knowledge of some European countries' intention to also attack Belarus.