They took part in a first meeting aimed at approving an agenda proposed by the current chair of the council, Tunisia.
The new countries will hold non-permanent seats on the 15-member council for two years.
The permanent members — the United States, Russia, China, France, and Britain — hold veto power.
Divisions within the council have sharpened in recent years as the US under President Donald Trump took a unilateral approach to diplomacy.
While veto power by any of the permanent members was used 10 times or fewer from 1990 to 2000, it was used on 31 occasions from 2010 to 2019, and five times last year alone, according SC Procedure, a specialized news outlet.