Rebels loyal to South Sudan's former vice president Riek Machar have reportedly seized Bor after days of fighting with government troops. Both sides are due to begin peace talks in the Ethiopian capital Addis Ababa today.
Uganda's president has warned rebel forces loyal to former South Sudan vice president that the region will unite to "defeat" him if he rejects a ceasefire offer.
Speaking in South Sudan's capital Juba, Yoweri Museveni said a regional bloc known as IGAD had given Riek Machar, who is accused of mounting a failed coup, "four days to respond" to the government's offer.
"If he doesn't we shall have to go for him, all of us," he said.
A meeting of East African leaders said last week it "welcomed the commitment" by South Sudan's government to cease hostilities against rebels and urged both sides to start peace talks by today.
South Sudan's army today fought with "White Army" ethnic militia, accusing rebels of mobilising the force despite its offer of a truce to end the conflict in the new country.
The White Army - made up largely of Nuer youths who dust their bodies with ash - clashed with government troops 18 miles from the town of Bor five days after rebels were driven out, Information Minister Michael Makuei said.
A rebel spokesman denied the White Army was controlled by Riek Machar, the former vice president whose followers oppose President Salva Kiir.
Foreign Secretary William Hague said he spoke to South Sudan's President Salva Kiir and his former vice-president Dr Riek Machar urging them to commit to a ceasefire and a negotiated settlement.
In a statement, the Foreign Secretary called:
"I have spoken today to President Salva Kiir and Dr Riek Machar to urge them both to commit to an immediate ceasefire. This is vital to prevent the deaths of more innocent civilians and to enable international humanitarian aid to reach people in desperate need.
"There can be no military solution to this conflict. I confirmed the UK’s full support for the negotiations proposed by African leaders, and I encouraged President Kiir and Dr Machar to enter into negotiations immediately and without preconditions.
"The UK stands ready to provide diplomatic support to those talks.
"I attended the independence celebrations of South Sudan in Juba in 2011 and witnessed the hope and optimism felt by its people after years of conflict and sacrifice. Their safety and their future must be put first.
A worsening conflict that causes South Sudan to slip back would be a tragedy."
Sudan Sudan troops will attack the main stronghold of rebel forces loyal to Vice President Riek Machar if the government's offer of a ceasefire is rejected.
The threat comes after the government offered a ceasefire and said it would release eight of 11 senior politicians, seen as allies of Machar, who were arrested over an alleged coup plot against President Salva Kiir.