The United Nations Special Coordinator for the Middle East Peace Process, Mr. Nickolay Mladenov, called upon political, religious and community leaders to speak out and to act decisively against those who incite and perpetrate hatred on all sides. ''The UN is determined to work with all parties to create the conditions which would enable a return to meaningful negotiations to finally resolve this conflict and bring sustainable peace to the people of Israel and Palestine," Mr. Mladenov said in a statement last night on the death of Reham Dawabsha, the third victim of the 31 July terrorist attack in Douma village which killed her husband and infant son, and wounded her four-year old son Ahmed. ''Our thoughts and prayers are with the family and especially with little Ahmed, now orphaned due to this heinous act.'' "I am deeply saddened by the death of Reham Dawabsha on the eve of her 27th birthday. She sadly succumbed to her wounds. ''I reiterate and strengthen my earlier call for justice. Acknowledging the wide condemnations issued at the time of the incident by Israeli and Palestinian leaders, I am nevertheless concerned by the lack of progress in identifying and prosecuting the perpetrators of this outrage.'' ''The tragedy of the Dawabsha family serves as a shocking example of the destructive nature of extremism,'' he added.