A bombing attack on a Medecins Sans Frontieres (MSF) hospital and pharmacy in South Sudan over the weekend was deliberate and may amount to a war crime, the UN Commission on Human Rights in South Sudan said on Tuesday.

MSF said on Saturday that at least seven people had been killed and 20 wounded in South Sudan's Fangak county when a bomb was dropped on the pharmacy, burning it to the ground and damaging the hospital, followed by another drone attack on Old Fangak, a town in the Greater Upper Nile region.

No one claimed responsibility for the attack, which came on the heels of air strikes and heavy clashes in the region in recent months between national forces and an ethnic militia allegedly allied with First Vice President Riek Machar.

"This was not a tragic accident. It was a calculated, unlawful attack on a protected medical facility," Yasmin Sooka, chair of the commission, said in a statement.

"The aerial bombing of the MSF hospital in Old Fangak is a flagrant violation of international humanitarian law and could amount to a war crime ...

"Targeting medical facilities and services violates the Geneva Conventions and represents a direct assault on foundations of humanitarian action that are intended to protect civilians in conflict zones."

Information Minister Michael Makuei did not respond to requests for comment.

South Sudan has officially been at peace since a deal in 2018 ended a five-year civil war between forces loyal to President Salva Kiir and fighters loyal to Machar.

But the house arrest of Machar in March on accusations of trying to start a rebellion has sparked international concern that conflict could reignite.