South Sudan and Uganda must investigate the use of banned cluster bombs in the recent fighting for the town of Bor, Human Rights Watch said Saturday.

United Nations experts found remnants of the bombs, including unexploded "bomblets", earlier this month close to Bor, a town that has changed hands four times in the latest fighting in South Sudan that started in mid-December.

The experts said they were found in an "area not known to be contaminated by remnants prior to mid-December 2013."

Cluster bombs, so called because they consists of clusters of scores or hundreds of submunitions or bomblets, are particularly deadly to civilian populations because many of the bomblets do not explode immediately on impact but lie hidden only to explode months or years after a conflict is over, often when handled by children.

"The young nation of South Sudan has enough problems without these horrific weapons, which kill and keep on killing long afterwards," said Steve Goose, arms division director at New-York-based HRW.

"The governments involved should quickly find out who is behind this and make clear they will be held responsible," added Goose.

Since mid-December Bor, the Jonglei State capital, and the area around it have changed hands several times in clashes between government forces, the Sudan People's Liberation Army (SPLA), and opposition forces loyal to former vice president Riek Machar.

The Ugandan military has been supporting South Sudan's government forces, including with air power.

The remnants found by the UN experts were of cluster bombs of the RBK-250-275 type, manufactured in the former Soviet Union, dropped by aircraft or helicopters.

UN chief Ban Ki-moon has condemned the use of cluster bombs in South Sudan, but has not indicated who the UN believes was responsible.

South Sudanese and Ugandan forces possess the air power to deliver air-dropped cluster munitions, while the opposition forces "are not believed to possess the means necessary to deliver these bombs," HRW said.

South Sudan government forces use the Mi-17 Hip helicopter, which is capable of dropping cluster munitions. However, there were no reports or allegations of any use of cluster munitions by the SPLA in the past when it functioned as a rebel armed force, the group said.

South Sudan's defence minister Kuol Manyang Juuk has strongly denied that any cluster munitions have been used, telling the Citizen newspaper: "It is not true, cluster bombs have never been used in the fighting."

Uganda's top military officials likewise deny having used cluster bombs, or indeed any bombs in the South Sudan conflict. Their subordinates on the ground have however told media they did carry out bombing raids.

HRW said it has evidence that the type of cluster bomb found near Bor was used at some point during the years of fighting between the Ugandan military and the Lord's Resistance Army rebels.