A dusk-to-dawn curfew was imposed in the South Sudan capital of Juba following clashes that included attacks on military barracks, officials said. Witnesses said about 21 people were killed but there was no official report on the number of casualties after the fighting Sunday. Military officials said forces from the former joint integrated units came under heavy gunfire after changes were instituted in the presidential guard force, the Sudan Tribune reported Monday. "The fighting was sparked off after forces predominantly from one ethnic group were deployed under the directive of Maj. Gen Marial Cinduong Yol, the commander of the presidential guard force," one official said. President Salva Kiir accused his former vice president, Riek Machar, for the attack on the barracks. Kiir sacked Machar during the summer. "These soldiers who attacked military barracks are aligned to Riek Machar," the president said. "Justice will prevail and those attackers are being pursued." Unconfirmed reports indicated the fighting erupted after Kiir allegedly ordered the army's chief of staff, James Hoth Mai, to arrest dissident politicians, the Tribune said. "This situation could have been contained since yesterday but the differences within the security and the army could not allow the taking of immediate action," a senior government official told the Tribune Monday.