Ntoumi, leader of the Conseil National de Résistance (CNR), a former rebel group turned political party, had on 25 April been appointed head of humanitarian affairs in the government.
The CNR said on Friday it had been told Ntoumi would instead be councillor at the convention for peace and reconstruction.
"CNR was informed during a meeting presided over by the prime minister on 8 May of the change in Pastor Ntoumi's status. CNR would like to let national and international opinion know that the government has failed to respect its own signature," CNR said in a statement.
"While confirming its commitment to peace and its willingness to work towards the consolidation of the ideal, CNR urges the government to return to the spirit and letter of the 25 April 2007 agreement," it added.
The 25 April deal was the first time the CNR and the government had signed a 'direct agreement' since 17 March 2003, when both parties reaffirmed the peace agreement. This facilitated an end to the hostilities between the 'Ninja' fighters of the CNR and the regular army in the Pool region between 1998 and 2002.
The new deal had also called for the destruction of arms belonging to Ntoumi's fighters, as well as the integration of 250 members of his militia into the national army.
Ntoumi's movement, which he has transformed into a political party since February, has asked the government to form an independent electoral commission. He has also asked for an electoral census in the areas of the Pool that had not voted since 2002.