Abdel-Salam al-Kubaisi, a spokesman for the Association of Muslim Scholars (AMS), the highest Sunni authority in Iraq, told a news conference on Saturday: "The association finds itself forced to reconsider the decisions reached at the Cairo conference.
"What is happening on the ground differs completely from what was promised."
Last month, leaders from across Iraq had reached a tentative agreement at talks in Cairo that violence should stop, some detainees should be freed and the US forces should gradually withdraw.
They had also agreed to hold further, more substantial, talks in Iraq next spring.
Al-Kubaisi's comments came ahead of parliamentary elections on 15 December that the US-backed Iraqi government hopes will draw fighters away from violence and to the ballot box.
The AMS has denounced violence, but many of its goals are similar to those of Sunni fighters who are waging a bloody campaign to topple the US-backed government led by Shias and Kurds.
If the group rejects reconciliation efforts, it is likely to hurt government efforts to persuade Sunni fighters to join the political process.
Delegates at the Cairo meeting
on Iraq at the end of November
Al-Kubaisi made the remarks after displaying pictures of a dead man and his child who he said were killed by the Scorpion Forces of the Shia-led Interior Ministry.
It was the latest charge made by the association, which has repeatedly accused the government of condoning hit squads that it says tortures and kills Sunnis.
The government denies the accusations.
The value of the Cairo accord was uncertain with the absence of anyone representing Sunni fighter groups.