JUBA — South Sudan will free four high-profile political prisoners facing treason charges, a presidential spokesman said on Thursday, meeting a demand by rebels in a faltering peace process.

President Salva Kiir has come under mounting pressure as rebel fighters loyal to Kiir's sacked deputy, Riek Machar, seize territory and close in on northern oil fields that provide the country's economic lifeline.

Kiir sacked his army chief on Wednesday, replacing him with a loyalist from his own ethnic group as the country's four-month conflict appears increasingly fought along ethnic lines.

Kiir had accused the four detainees - a former ruling party official, national security minister, deputy finance minister and ambassador to Washington - of fomenting a coup when fighting erupted between soldiers loyal to him and his rival Machar in mid-December. They faced nine charges each.