The U.N. children's agency says dozens of children have been killed, at least 12 raped and others abducted and recruited in a series of attacks in South Sudan's Unity state over a two-week period.

UNICEF said in a statement Monday that witnesses believe the attacks were carried out by armed groups aligned with South Sudan's military.

South Sudan's army recently launched an offensive against rebel forces, drawing concern from watchdog groups and humanitarian organizations.

Doctors Without Borders reported Sunday that health workers had fled with their patients into swamps as clashes hit the rebel stronghold of Leer in Unity state.

The International Committee of the Red Cross warned Monday that many people forced to flee fighting will suffer from a lack of food and health care while on the run.