The United States on Thursday reaffirmed its vow to cut aid to violence-wracked South Sudan if the government of President Salva Kiir is overthrown in a coup.

Troops loyal to Kiir have been battling forces allied to former vice president Riek Machar in a wave of ethnic violence that has left thousands dead, according to the United Nations.

South Sudan's army battled rebel forces Wednesday in one key town while troops flushed out insurgents in another after its recapture, as the U.N. moves to double its peacekeeping force to stave off civil war.

Thousands are believed to have been killed in more than a week of violence, with reports of bodies piled in mass graves amid escalating battles between troops loyal to President Salva Kiir and those backing his rival Riek Machar, a former vice president who was sacked in July.

The U.S. military on Tuesday deployed a small team of marines to Uganda to prepare for possible further evacuations of Americans from violence-wracked South Sudan, officials said.

Pentagon spokesman Colonel Steven Warren said a "platoon-sized" contingent of marines and a C-130 aircraft had been detached from a deployment in Djibouti and sent to Entebbe, Uganda.

Sudanese President Omar al-Bashir on Tuesday said he was "deeply worried" about the worsening violence in neighboring South Sudan and the Central African Republic.

Fighting has gripped South Sudan for more than a week, after President Salva Kiir accused his former deputy Riek Machar, who was fired from the government in July, of attempting a coup.

The U.N. humanitarian chief in South Sudan warned Sunday the country has "unraveled" after a week of violence that has transformed a power struggle within the ruling party into a quasi-civil war.

The world's youngest nation, which gained independence from Sudan in 2011 after decades of combat, is falling apart under the pressure of rebel groups seizing towns and oil fields and militias from the country's two largest tribes massacring one another after a fall-out between the President Salva Kiir and his former deputy Riek Machar.

Sudan's President Omar al-Bashir has expressed concern about instability in neighboring South Sudan and other volatile states in the region, official media reported Sunday.

Bashir "expressed his concern about what is going on in South Sudan, Central African Republic, Libya and Egypt,” the state SUNA news agency reported.

Brutal fighting in South Sudan has reopened deep-rooted ethnic divisions, forcing tens of thousands of terrified residents to seek shelter at U.N. bases or flee in fear of attacks.

"I am afraid," said Susan Nakiden, who still has bitter memories of the more two decades of civil war that paved the way to the young nation's independence two years ago, when when she was forced to flee her home.

Kenya said Saturday it will send troops to troubled South Sudan to evacuate some 1,600 citizens, with many trapped in the flashpoint rebel-held town of Bor.

President Uhuru Kenyatta has "ordered the KDF (Kenya Defense Force) to commence immediate evacuation of the 1,600 Kenyans stranded in South Sudan", presidential spokesman Manoah Esipisu said in a statement.

Sudan's military says it has "liberated" several areas of war-torn South Kordofan state, but rebels on Saturday dismissed the claim as propaganda, saying a government offensive achieved "nothing".

The Sudanese Armed Forces (SAF) said late Friday that it had taken over Abu Hassan, a rebel command center, "after a series of victories" elsewhere in the state.

Sudan expressed fears Friday over the fate of vital oil flows and an influx of refugees and weapons, as fighting between rival army factions spread in South Sudan.

"This will affect all the neighboring countries. In Sudan we are going to suffer even more than the others," Information Minister Ahmed Bilal Osman told Agence France Presse.