Igor Azarenok, commander of the Air Force and Air Defense Forces, said the countries' defense ministries were currently discussing delivery terms.
The S-400 is designed to intercept and destroy airborne targets at a distance of up to 400 kilometers (250 miles), twice the range of the U.S. MIM-104 Patriot, and 2 1/2 times that of the S-300PMU-2.
The system is also believed to be able to destroy stealth aircraft, cruise missiles and ballistic missiles, and is effective at ranges up to 3,500 kilometers (2,200 miles) and speeds up to 4.8 kilometers (3 miles) per second.
Azarenok also said the Russian and Belarusian parliaments would ratify the integrated air-defense agreement in May.
He said Belarus would modernize its Air Force by buying new warplanes from Russia and upgrading others.
The official said the Belarusian Air Force and Air Defense Forces had plans to conduct over 200 various drills, exercises, and field training sessions.
Moscow and Minsk signed an agreement in February on the joint protection of the Russia-Belarus Union State's airspace and the creation of an integrated regional air defense network.
It will comprise five Air Force units, 10 anti-aircraft units, five technical service and support units and one electronic warfare unit, and will be placed under the command of a Russian or Belarusian Air Force or Air Defense Force senior commander.