It accused the government troops of committing war crimes and possibly crimes against humanity during its offensive.

"Government-aligned forces carried out gruesome killings and widespread rapes and burned countless homes as they swept across large parts of Unity State," Daniel Bekele, Africa director at Human Rights Watch, said.

The conflict in South Sudan began in December 2013, when clashes erupted between forces loyal to President Salva Kiir and those aligned to the former Vice President Riek Machar.

Two million people have fled their homes, with 100,000 forced to flee since May alone.

The struggle between the two sides has been dominated by war crimes, including widespread killings of civilians because of their ethnicity or perceived allegiances, HRW said.

The South Sudanese army and their allies in the Bul Nuer ethnic group forcibly displaced tens of thousands of people in Unity by attacking women, elderly, and children, while burning villages and stealing cattle, Human Rights Watch alleges.

The group said many of the attacks were against villages with no presence of rebel fighters.

-Al Jazeera