Fighting also erupted outside the flashpoint town of Bor, capital of vast Jonglei state which has untapped oil reserves.
US Secretary of State John Kerry on Sunday voiced his support for the Addis Ababa peace talks and warned against the use of force by either side to gain the upper hand, reports Reuters.
“The negotiations have to be serious. They cannot be a delay gimmick in order to continue the fighting and try to find advantage on the ground at the expense of the people of South Sudan,” Kerry told reporters during a visit to Israel.
Rebel and government negotiators were supposed to sit down face-to-face for the first time on Sunday. But the rebel delegation and a Western diplomat told Reuters late in the evening there would be no meeting that day.
Kiir blamed his long term rival, whom he sacked in July, for starting the fighting in a bid to seize power. Machar dismissed the allegation but he has acknowledged leading soldiers battling the government.
A key stumbling block to the talks is what should happen to a number of political detainees allied to Machar who are accused of involvement in the plot.
The rebels have demanded their comrades’ release - a call backed by the United States and European Union.
“This is a capital offence, it is a case of treason and we are expected as the government of the Republic of South Sudan to investigate within two, three days? This is out of the question,” South Sudan's Information Minister Michael Makuei told reporters in Addis Ababa.
Several false starts have dampened hopes for a swift end to the fighting, which has driven more than 200,000 people from their homes. The United Nations is scrambling to raise money to provide food, clean water and shelter.
Sudan’s state news service reported Sudanese President Omar Hassan al-Bashir would head to Juba on Monday to meet Kiir.