Adrian Severin, the UN special rapporteur on Belarus, said the legislation could "severely undermine" fundamental freedoms.
The changes were passed by Belarusian lawmakers on 8 December. They would make it a crime punishable by up to two years in jail to "discredit Belarus" in the eyes of international organizations and foreign governments.
The same jail terms would apply to people convicted of distributing "false information" on the political, economic, social, or international situation of Belarus, or the legal situation of its citizens.
The legislation must be signed into law by President Alyaksandr Lukashenka.