Mr Kiir has been in power since South Sudan's independence in 2011 and elections in 2015 were also postponed.

The move could undermine ongoing peace talks aimed at ending almost five years of civil war in the country, says BBC World Service Africa editor Will Ross.

Last month Mr Kiir signed a deal with his rival Riek Machar in which both agreed to a permanent ceasefire.

The agreement was supposed to be followed by further negotiations and the setting up of a power-sharing government - with Mr Machar reappointed as a vice-president and other opposition politicians also taking up positions in an expanded cabinet.

Mr Kiir's critics accuse him of being an authoritarian ruler clinging to power. He denies the allegation.The civil war was sparked in 2013 because of rivalry between Mr Kiir and this then-deputy Mr Machar, who had ambitions to succeed him.