South Sudanese rebel troops have seized control of Jonglei state capital Bor, a spokesman has said.

Moses Ruai Lat, based in the northern state of Unity, claimed that rebel forces were now marching towards the country's capital Juba and were already "close" to the city.

But the Sudan People's Liberation Army (SPLA), which is loyal to the government, countered that its forces were advancing on Bor, which is 118 miles north of Juba, to meet rebel militias heading south.

Such reports are raising concerns of a further escalation in violence that has already killed 1,000 people in South Sudan - a country the size of France with nearly 11 million people that seceded from Sudan in 2011.

President Salva Kiir has accused long-term political rival and former vice-president Riek Machar, whom he sacked in July, of starting the fighting in a bid to seize power. Dr Machar denies the claim.

Meanwhile, peace negotiations being held in the Ethiopian capital Addis Ababa got off to a slow start.

"Both delegations are meeting the mediators separately," said Dina Mufti, a spokesman for Ethiopia's foreign ministry.

"We hope to bring both sides into face-to-face talks soon."

He said that the introductory meetings were necessary to bridge the groups' differences ahead of direct talks which are expected to start tomorrow at Addis Ababa's Sheraton Hotel.

President Kiir's government and the rebels loyal to Dr Machar have agreed in principle to a ceasefire, mediators say, but there is no agreement yet on a starting date.

Diplomats say that both sides still seem more interested in military advantage than driving the talks forward.

The US has evacuated more of its embassy staff from Juba due to worsening security in the country.

"We are not suspending our operations. We are just minimising our presence," claimed US ambassador to Juba Susan Page.

An emergency message to US citizens on the embassy's website said there would be an evacuation flight today arranged by the US State Department.

The South Sudanese government has declared a state of emergency in the states of Unity and Jonglei.