WASHINGTON - US President Barack Obama has dispatched 45 US troops to South Sudan to protect US nationals and embassy there amid the African nation's volatile security situation.

In a letter sent to Congress on Thursday, Obama said the troops left for South Sudan on Wednesday, and would remain there until the situation improves.

Meanwhile, more US citizens were evacuated from South Sudan on Thursday, as government troops and rebel forces continued to clash.

A private charter flight lifted some 130 persons from capital Juba, among whom were US citizens and third-country nationals, State Department Spokeswoman Marie Harf told a regular news briefing.

She said a small number of Americans also flew out of the country aboard a British military aircraft.

A day earlier, Washington airlifted three groups of American nationals, including non-emergency diplomatic personnel, US citizens and third-country nationals, out of South Sudan.

Since fighting broke out on Sunday between forces loyal to President Salva Kiir and his former deputy Riek Machar, who was removed from office in July, some 450 people have been killed in clashes so far in Juba.

Machar has rejected Kiir's offer for talks, calling instead for the president's removal by the ruling party and the army.

Kiir accused Machar of orchestrating a military coup against him and declared on Monday a state of emergency in the country with a curfew imposed from 6:00 pm to 6:00 am local time.