"I had a frank and open discussion with President Salva Kiir," Booth, in Juba, told reporters in Washington.
"Importantly, President Kiir committed to me that he was ready to begin talks with Riek Machar to end the crisis without preconditions as soon as his counterpart is willing."
Booth, who is also the US special envoy for Sudan, said he met with a group of 11 senior figures in the ruling Sudan People's Liberation Movement "who remain detained in Juba."
"I can report that they are secure and well taken care of," Booth said.
"These individuals communicated to me their desire and their readiness to play a constructive role... For peaceful political dialogue and national reconciliation."
South Sudan's army today was poised for a major offensive against rebel forces, as the country slid towards civil war despite international peace efforts.
The UN Security Council was set to hold emergency talks on the situation later Monday, at 2200 GMT. UN Secretary General Ban Ki-moon has called for the reinforcement of the UN mission in South Sudan.
