Machar's ouster from the country's No. 2 political position earlier this year had stoked ethnic tensions.
Machar, who has criticized Kiir as a dictator, later said he would contest presidential elections in 2015.
U.N. Secretary-General Ban Ki-moon on Sunday urged South Sudan's leaders “to do everything in their power” to stop the violence.
Foreign ministers from neighboring countries Kenya, Ethiopia, Uganda and Djibouti were in South Sudan earlier in the week to try and diffuse the crisis.
South Sudan, which became independent in 2011 after decades of a brutal war with Sudan, has been plagued by ethnic discord, corruption and conflict with Sudan over oil revenues.
Although the south inherited three-quarters of Sudan's oil production when it declared independence in 2012, its oil exports are pumped through pipelines running north, raising concern a rebel takeover of southern oil fields could invite Sudan into the conflict.
