UNITED NATIONS — An unpublished report from the United Nations peacekeeping force in southern Sudan gives details of violence that has erupted in an important border state, including widespread aerial bombardments that kill civilians, executions, possible mass graves and attacks on churches.

The report emerged as the newly minted Republic of South Sudan, which officially seceded from the northern part of the country on July 9, was admitted to the United Nations on Thursday. Its designated ambassador, Ezekiel Gatkuoth, broke out a few jubilant dance moves as the new country’s striped flag, with its distinctive yellow star, rose alongside those of the other 192 member states.

In speeches, representatives from both Sudan and South Sudan pledged to work peacefully to untangle the remaining complex issues, including demarcating the border between them, sharing oil and determining citizenship.

“We and our brothers in South Sudan have left bitterness and the wounds of war behind us, and we’re looking forward to the future,” Daffa-Alla Elhag Ali Osman, the Sudanese envoy to the United Nations, told the General Assembly.