The Holy See is refusing due to the "special nature" of the council, which is clearly not based on the participation of states, Vatican Secretary of State Pietro Parolin said, according to Vatican News.
One concern is that the Peace Council could challenge the role of the UN.
At the international level, it should primarily be the UN that handles these crisis situations, Parolin said.
The original idea of the council was to monitor the ceasefire between Israel and Hamas, and to oversee the reconstruction of Gaza.
Later, Trump, who is the chairman with veto power over all decisions, has invited more actors.
Several Western countries, including Sweden, have rejected membership. The first meeting will be held on Thursday.
