WASHINGTON - South Sudan President Salva Kiir said he was ready to begin talks with former Vice-President Riek Machar "without preconditions" to end the current crisis in the conflict-ravaged country, a US envoy said Monday.

US special envoy to Sudan and South Sudan, Donald Booth, said he had a "frank and open" discussion with Kiir in the South Sudanese capital of Juba earlier in the day.

"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 was willing," Booth told reporters via teleconference.

Deadly clashes have escalated in South Sudan since Dec 15, following what Kiir's government claimed was an attempted coup by soldiers loyal to Machar, who was dismissed in July.

Booth said he had met with a group of 11 senior figures in the Sudan People's Liberation Movement "who remain detained in Juba" and were "secure and well taken care of." They were detained by Kiir's government last week in connection with an investigation into the alleged coup attempt.

The detained figures expressed "their desire and their readiness to play a constructive role in ending the crisis through peaceful political dialogue and national reconciliation", he added.

The US welcomes the recent political engagement of the Intergovernmental Authority on Development in South Sudan, Booth said, adding that he encouraged the regional mediation bloc to move swiftly in engaging the government and opposition forces.

"The US emphasizes the urgency of the situation and stands ready to support these efforts as necessary," Booth said.

The US State Department said on Sunday some 380 US officials and citizens as well as about 300 citizens of other countries had been airlifted out of South Sudan.

The Pentagon said Monday that US military is redeploying aircraft and other forces in the Horn of Africa to prepare for possible further evacuations of US citizens from South Sudan.

The US forces are repositioning forces in the area and units are ready for orders of evacuating more US citizens on the ground, Pentagon spokesman Colonel Steven Warren told reporters.