The announcement in a late night broadcast by the state-run South Sudan Television said seven of the 10 previous governors have been reappointed along with 21 others to run the new administrative zones.
Kiir's move to create 28 states comes after 150 rebels returned to the capital Juba on Monday to implement a peace deal signed by Kiir and rebel leader Riek Machar.
The rebel leadership is consulting on their reaction to the president's action, Machar's spokesman James Gatdet Dak told The Associated Press today.
The decree is unconstitutional because it violates the signed peace agreement and because the parliament, not the president, has the power to change state borders, said Lam Akol, the head of South Sudan's official opposition, the SPLM-Democratic Change party.
The new states divide South Sudan along tribal lines and the opposition party will not recognise the new states, charged Akol.
The president's action is "unfortunate" because it presents a "key challenge" to implementing the peace agreement, said the Juba-based activist group Community Empowerment for Progress Organization in a statement today.
South Sudan's war began in December 2013. Tens of thousands have been killed in the fighting which has displaced more than 2 million from their homes.
