(Recasts throughout with quotes from envoy, background on conflict)
WASHINGTON, Dec 23 (Reuters) - South Sudan's President Salva Kiir said on Monday he is committed to beginning talks with rebel leader Riek Machar, the country's former vice president, to end violence, a senior U.S. official said after meeting Kiir in the capital 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 is willing," U.S. special envoy to South Sudan, Donald Booth, told a conference call with reporters.
It was not immediately clear how soon a meeting could be arranged.
Booth was sent to South Sudan at the weekend to seek a diplomatic solution to avoid the violence spiraling into an ethnic civil war.
Earlier, Machar told Reuters he was ready for dialogue but Kiir must first release his detained political allies.
The violence began last week when Kiir, a member of the majority Dinka ethnic group, accused Machar, a member of the rival Nuer ethnic group, of attempting to seize power by force. Kiir also detained 11 politicians, some of them former ministers, in connection with the "foiled coup".
Booth said he visited the detained officials who were "secure and well taken care of."
"These individuals communicated to me their desire and their readiness to play a constructive role in ending the crisis through peaceful political dialogue and national reconciliation," he added.
Diplomats said U.N. chief Ban Ki-moon would seek to boost the number of peacekeepers in South Sudan to better protect civilians from the escalating conflict.
The UN Security Council is expected to adopt a resolution on Tuesday approving about 5,000 troops and 280 more police for the UN peacekeeping mission, known as UNMISS, the diplomats said.
Meanwhile, U.S. officials said the U.S. military was deploying about 150 Marines to a base in Djibouti in case further evacuations were needed.
A senior U.S. administration official said it was hard to know exactly how many Americans were still in South Sudan because many did not register with the U.S. embassy.
An estimated 380 American citizens have so far been evacuated from the country, the official said.
On Saturday, four U.S. service members were wounded when their military aircraft was hit by ground fire as they tried to evacuate U.S. citizens from the town of Bor. The U.S. citizens and other foreigners were later evacuated by helicopter from the same area a day later. (Reporting by Lesley Wroughton; Editing by Chizu Nomiyama)