The UN children's agency said this morning it fears hundreds of boys were kidnapped last month in the northeast of South Sudan, raising suspicions that the abductors were from a pro-government militia.
Last week UNICEF had estimated that 89 boys, some as young as 13, were abducted by an armed group in Wau Shilluk.
The riverside town is in government-held territory within the oil-rich Upper Nile state.
"The organisation now believes the number of children may be in the hundreds," UNICEF said in a statement.
It added that the suspected kidnappers were from a "militia... aligned with the government's SPLA forces."
Witnesses to the mass abduction on 15-16 February said that unidentified armed soldiers surrounded the community and went house to house taking away by force any boys thought to be over 12.
"The recruitment and use of children by armed forces destroys families and communities," Jonathan Veitch, the head of UNICEF in South Sudan, said when the kidnappings were first reported.
UNICEF estimates there are at least 12,000 children used by both sides in South Sudan's ongoing civil war.
South Sudan plunged into violence in December 2013 when fighting erupted between forces loyal to President Salva Kiir and rebels led by his rival, former vice president Riek Machar.
Each side has accused the other of forcibly recruiting child soldiers.
The president's spokesman Ateny Wek Ateny had strongly condemned last month's kidnappings, saying: "The government of South Sudan will not tolerate the use of our children for violence."
"This is the equivalent to the Boko Haram of South Sudan," he said, referring to the Islamist extremist group behind an insurgency in northern Nigeria that has abducted boys and girls.
But after two weeks of inquiries, UNICEF said it believes government-aligned militia chief Johnson Olony, who controls the area and has been accused in the past by Human Rights Watch of using child soldiers, is responsible for the kidnappings.
The UN agency added that the South Sudanese army does not control this militia group.
Tens of thousands of people have died in the conflict, 1.5 million have been displaced and 2.5 million are in dire need of food aid in South Sudan, which declared independence from Sudan in 2011.