WASHINGTON -?US Secretary of State John Kerry announced Friday that he has sent a special envoy to South Sudan to support the efforts to end the violence in the African country.

US Special Envoy for Sudan and South Sudan Donald Booth departed ?Friday to "support regional efforts already underway" to end the armed conflict in South Sudan, Kerry said in a statement.

Kerry said he called South Sudanese President Salva Kiir Thursday night to urge him to protect all South Sudanese citizens and work toward reconciliation.

"Now is the time for South Sudan's leaders to rein in armed groups under their control, immediately cease attacks on civilians, and end the chain of retributive violence between different ethnic and political groups. The violence must stop, the dialogue must intensify," Kerry said.

Kerry strongly condemned Thursday's attacks on the United Nations Mission in South Sudan (UNMISS) base in Akobo County in Jonglei State, while urging all parties to respect UNMISS, refrain from any attacks on its personnel, and help facilitate its mission to protect civilians.

The top US diplomat warned that any armed attack on the South Sudanese capital of Juba "will be seen as an attempt to achieve an unlawful usurpation of power, which would be universally condemned."

"Those who seek to take or hold power by violence or division of South Sudanese along ethnic lines will not have our support. Violence today will not pave the way for a more stable or prosperous tomorrow," Kerry said.

With violence escalating in South Sudan, US President Barack Obama on Wednesday deployed 45 US troops to the African country to protect US citizens and property. The US government on Thursday evacuated more citizens from South Sudan, as a private charter flight lifted some 130 persons from Juba.

Since fighting broke out on Sunday between forces loyal to President Kiir and his former deputy Riek Machar, who was removed from office in July, some 450 people have been killed so far in Juba. Meanwhile, Machar has rejected an offer of talks by Kiir, calling instead for the president's removal by the ruling party and the army.