JUBA, June 30 (Reuters) - The United Nations accused South Sudan's government forces and its allies on Tuesday of sexually abusing women and girls and reportedly burning some alive in their homes during recent fighting in the conflict-torn nation.

The U.N. Mission in South Sudan, or UNMISS, said the findings came from a report based on interviews with 115 victims from districts in oil-rich Unity State, where the government's SPLA forces launched an offensive against rebels in late April.

South Sudan, the world's newest nation and one of its poorest, has been mired in a conflict since December 2013 that pits President Salva Kiir's SPLA against those loyal to rebel leader Riek Machar, a former deputy president.

"The survivors of these attacks reported that SPLA and allied militias from Mayom County carried out a campaign against the local population that killed civilians, looted and destroyed villages and displaced over 100,000 people," UNMISS said.

In a statement outlining findings from its report, UNMISS said its rights officers had focused on the abduction and sexual abuse of women and girls, "some of whom were reportedly burnt alive in their dwellings."

"This recent upsurge (in fighting) has not only been marked by allegations of killing, rape, abduction, looting, arson and displacement, but by a new brutality and intensity," it said.

The report added that the "level of cruelty" reflected more than political differences.

The fighting has increasingly turned along ethnic lines, with Kiir's Dinka ethnic group battling Machar's Nuers.

UNMISS said it had sought to visit sites of the alleged atrocities to verify allegations but said it was routinely denied access by the SPLA, and also encountered logistical obstacles that it did not specify.

Reuters could not reach an SPLA official for comment.

UNMISS said it gave its report to the Foreign Ministry before release but officials had not commented. It said the SPLA's spokesman had dismissed such allegations in the past but had also welcomed investigations.

There have been periodic flare-ups in the conflict during the past year and a half, usually picking up in drier months when it is easier for troops to move around a nation that is the size of France or Texas yet with almost no tarmac roads.

(Additional reporting by Tom Miles in Geneva; Writing by Edmund Blair; Editing by Mark Heinrich)