GENEVA — Sri Lanka’s new government said Monday that it would set up a truth, justice and reconciliation commission and draft a new constitution to stabilize the country and address the bitter grievances left by its decades-long civil war.

The foreign minister, Mangala Samaraweera, announced the measures in a speech to the United Nations Human Rights Council here. His address came three days before the release of a long-awaited United Nations report on the killing of an estimated 40,000 ethnic Tamil civilians in 2009 by armed forces under President Mahinda Rajapaksa.

The report was supposed to have been presented to the council in March but was delayed to give President Maithripala Sirisena, who had defeated Mr. Rajapaksa in an election two months earlier, time to come up with plans for achieving reconciliation and accountability and for cooperating with international investigators.

According to Mr. Samaraweera, the government planned to set up the reconciliation commission with advice from South Africa. It also proposed the creation of an Office of Missing Persons to identify the fate of people who disappeared during the civil war, and an Office of Reparations to address compensation. It also planned to create a “constituent assembly of Parliament” to prepare a new constitution in order to avoid a recurrence of conflict, he said.