Mr Kiir authorised military to use force if armed civilians did not lay down their weapons peacefully.
South Sudan's President Salva Kiir has declared a state of emergency in three states and ordered an immediate disarmament exercise.
In a statement released on Monday night, Mr Kiir ordered his military chiefs to mobilise enough forces and equipment to enforce a state of emergency for up to three months.
He also authorised them to use force if armed civilians did not lay down their weapons peacefully.
And he has political backing. Dhoruai Mabor Teny, an MP for a county in Western Lakes, had earlier encouraged the president to suspend the constitution and deploy the military.
At least 170 people have been killed in tribal clashes in one of the northern states - Western Lakes - in recent weeks.
The violence in Western Lakes has been between armed youths from two rival Dinka sub-clans which has involved the burning of houses and the destruction of property.
Previous clashes have been over scarce grazing for cattle and other cultural and political grievances. But the death toll in this latest outbreak has been much greater.