JUBA (Reuters) - The U.N. children’s agency UNICEF said on Tuesday three South Sudanese working for its contractor were detained last week, as aid workers are increasingly targeted in the four-year civil war.

Tim Irwin, UNICEF’s chief communications officer in South Sudan, said the three were working for Montrose, a firm contracted by UNICEF to conduct education surveys in Mathiang, a village in Pagak in the northeast.

He said on July 6 heavy fighting erupted in the area. “The three fled Mathiang along with other humanitarian actors and were detained in Pagak upon arrival,” he said.

Last month South Sudan’s government said it may prevent aid workers from traveling to some rebel-held areas on security grounds.

Irwin did not say who the agency believed had detained the workers but said UNICEF was pursuing efforts to “establish the exact location of the contractors, their state of health and to secure their freedom.”

Africa’s youngest nation was plunged into civil war in 2013 after President Salva Kiir, who hails from the Dinka tribe, sacked his deputy, Riek Machar, a Nuer. Attempts at a peace deal have failed to stem the fighting.

Lam Paul Gabriel, spokesman for Machar’s SPLA-IO rebel movement said the group had not detained the contractors.

“NGOs (non-governmental organizations) are always there working and helping the communities ... why would we abduct or arrest them?” he said.

A spokesman for the government-allied opposition faction, Agel Machar, said the aid workers, all men, were not part of the conflict and should be released immediately.

Aid agency staff have been increasingly targeted in South Sudan’s conflict, which has killed tens of thousands of people and displaced more than three million.

At least twelve soldiers are on trial by a military tribunal for the rape of five foreign aid workers.