Machar said he hopes a settlement will be reached, but that a number of sticking points were preventing the two sides from agreeing.
The rebels are demanding the release of 11 opposition politicians, who were arrested last month.
"I think that we were not closer to a deal because apparently they have played down the importance of releasing the political detainees," Machar told Voice of America.
The rebels have also taken issue with reports of the Uganda military supporting the South Sudan government forces.
But Machar said his team will stay committed to the negotiations.
"I'm interested in cessation of hostilities, after all I'm the underdog in the matter. Who would be interested more in cessation of hostilities?" he said.
The fighting began in Juba on Dec. 15. Nearly 400,000 people have been displaced.