JUBA: The United Nations’ chief in South Sudan called on Friday for the immediate release of 12 colleagues seized by rebel forces, warning their capture was a possible war crime.

Around 100 rebel fighters, who have been battling the government for almost two years of civil war, captured 30 members of the UN peacekeeping mission in South Sudan (UNMISS) on Monday.

Rebels freed 18 peacekeepers on Thursday but still kept 12 UN contractors they had seized on a barge in a contested area of the war-torn African country, UN officials said. All were on a river barge carrying fuel for the UN mission.

“I am adamant that the taking of UNMISS personnel as hostages is equivalent to an attack on the UN,” Ellen Margrethe Loj said Friday.

The rebels said they had taken the vessels because they were thought to be transporting weapons for government troops.Although South Sudan’s warring factions signed a peace deal in August, fighting persists, especially the contested states of Upper Nile and Unity.

On Monday, a UN barge resupplying fuel on the Nile River was blocked in the Kaka area by over 100 heavily armed men affiliated with rebels who detained the peacekeepers and the barge crew. The fuel and peacekeepers’ weapons were then looted, the UN statement said.

“UNMISS is extremely concerned that the national crew of the barge have not been released, the barge has not been returned to the United Nations, and neither has its equipment,” the UN statement said, urging rebel leader Riek Machar to free the contractors.

The barge was seized on the rebel-controlled west bank of the river. The east side, where the UN mission has bases, is held by government troops.

Two government soldiers, three South Sudan national security agency officials and six barge technicians are among those still in detention, said Nyagwal Ajak Dengkak, a local rebel spokesman. Rebel forces are taking “tough action against our enemies ... for the world to know the truth of the conspiracy between” the South Sudanese military and the UN mission, he said in a statement.

It was not possible to verify the rebels’ statement.

Twelve of the freed peacekeepers are from Bangladesh, a Bangladeshi official told The Associated Press.

“Such attacks against UN peacekeepers and other personnel may constitute a war crime.”

South Sudan’s army on Thursday claimed the UN barges had supplied fuel and “logistic items” to rebel general Gabriel Tanginya, a powerful warlord in the oil-zone areas of Upper Nile. But Loj rejected those accusations.

“All of the fuel cargo was intended for resupplying the UNMISS base,” she said.

Some 12,500 peacekeepers are deployed in South Sudan, which has been wracked by conflict since December 2013.

Tens of thousands of people have been killed, and UN-backed experts have warned of the “concrete risk of famine” before the end of the year, if fighting continues and aid does not reach the hardest hit areas.

Both sides are accused of having perpetrated ethnic massacres, recruited and killed children and carried out widespread rape, torture and forced displacement of populations to “cleanse” areas of their opponents.

Some 3.9 million people are in crisis, a third of the country’s population, a massive 80 percent rise compared to the same period last year, the UN said.