All foreign nationals are required to obtain work permits unless you are a national of the Russian Federation. A work permit is issued after your employer applies to the local Belarusian authorities for one. Your employer should have permission to employ a foreign workforce. The whole process for obtaining a work permit is generally between two to three weeks.
Once you obtained the work permit, it is necessary to apply for a work visa. A work visa is given only once you have your work permit; the work visa is valid for 90 days. It becomes necessary to get permission to live temporarily in the Belarus from the relevant immigration authorities, if you are there for work on a longer term.