Bashir, the subject of an arrest warrant issued in March 2009 by the Hague-based International Criminal Court (ICC) on suspicion of war crimes in Sudan's Darfur region, was elected president last month in a vote marred by charges of fraud.
Haile Menkerios, head of the UN Mission in Sudan, and Ibrahim Gambari, joint head of the African Union/UN Mission in Darfur, will attend the May 27 ceremony, spokeswoman Marie Okabe told reporters.
The United Nations says its leadership is under legal advice to keep contacts with Bashir to a minimum because of the arrest warrant.
But Okabe said Menkerios and Gambari "interact with the host government regularly on operational issues to ensure that their missions are able to function effectively and to address areas of mutual concern."
Another UN official, who asked not to be identified by name, said the two men were attending the inauguration as a "diplomatic courtesy."
In letters sent this week to the 111 countries that are parties to the ICC statute and to the United States, which is not a party, New York-based Human Rights Watch said they should not attend events with Bashir "unless absolutely essential."
"An inauguration is a ceremonial event that cannot be justified as an essential meeting," the letters said.
Human Rights Watch also quoted what it said was a legal opinion issued by the UN Office of Legal Affairs in 2006 as saying: "The presence of UN representatives in any ceremonial or similar occasion with (individuals indicted by international courts) should be avoided."
