JUBA, South Sudan (AFP) - South Sudan's army was poised for a major offensive against rebel forces, the President said on Monday, Dec 23, 2013, as the UN Security Council called for emergency talks in a bid to avert a slide towards civil war.

The French mission to the United Nations said the meeting, to begin at 5.00 pm (2200 GMT), would notably consider reinforcing the UN mission in the country (UNMISS).

UN chief Ban Ki Moon said earlier that he would ask for "additional troops, police and logistical assets", although he did not specify numbers.

The United Nations has warned that the situation in the world's youngest nation was fast unravelling, with hundreds of thousands of civilians now at risk.

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.

On Monday the US special envoy to the violence-wracked country, Mr Donald Booth, told reporters in Washington from Juba that Mr Kiir was ready for talks with Mr Machar "without preconditions".

In what Mr Booth described as a "frank and open discussion", he said Mr Kiir "committed to me that he was ready to begin talks with Riek Machar to end the crisis without preconditions as soon as his counterpart is willing".

Mr Kiir earlier said regional nations had offered to host talks.

Mr Machar has accused Mr Kiir of carrying out a vicious purge of his rivals.

Vowing to oust Mr Kiir, his forces have since seized the town of Bor, capital of the powder-keg eastern Jonglei state and located just 200km north of Juba, as well as the town of Bentiu, capital of crucial oil-producing Unity state.

The army is "now ready to move to Bor," Mr Kiir told Parliament, adding that the counter-attack was delayed until US citizens had been airlifted out.

The Pentagon said on Monday that aircraft and other forces were deploying to the Horn of Africa to prepare for possible further evacuations of Americans.

"We are repositioning our forces in the area of concern," spokesman Colonel Steven Warren said, adding however that no military action was under consideration.

Reports of a new offensive come despite days of shuttle diplomacy by African nations and calls from Western powers for the fighting to stop in the country, which won independence from Sudan just two and a half years ago, in July 2011.

The top UN humanitarian coordinator in South Sudan, Mr Toby Lanzer, said on Monday the situation was rapidly deteriorating.

"It would have been difficult one week ago to imagine that things would have unravelled to this extent," Mr Lanzer told Agence France-Presse. "There are hundreds of thousands of South Sudanese who've fled into the bush or back to their villages to get out of harm's way." Asked which areas of the conflict-torn country he was most concerned about, Mr Lanzer said that "it would be quicker to talk about which areas I'm not worried about" - an indication that an all-out civil war was now a real prospect.

"I hope to be wrong. Otherwise, hundreds of thousands will need help very soon," he said, admitting that UN peacekeepers were ill-equipped and lacking the numbers to protect civilians seeking shelter with them.

The clashes have left hundreds dead - probably many more. The European Union's aid chief Kristalina Georgieva said the country was "at the brink of a humanitarian tragedy".

The young nation is oil-rich but deeply impoverished and awash with guns after the long war with Khartoum, and has grappled with corruption and lawlessness since independence. There are both ethnic and political dimensions to the fighting, as troops loyal to Mr Kiir, an ethnic Dinka, battle forces backing Mr Machar, a Nuer.

Fighting has also spread to Upper Nile state, said Doctors Without Borders (MSF), where they treated 24 people for gunshot wounds on Sunday.

Mr Jok Madut Jok, of the Juba-based Sudd Institute think tank, reported heavy fighting as well in Upper Nile's state capital Malakal, although the government remained in control.

One student, an ethnic Dinka, called "in panic to report that he and the rest of Dinka students at the university... will surely be killed if Riek's (Machar's) forces take control of Malakal," Mr Jok said.

Nuer gunmen stormed a UN base last week killing two Indian peacekeepers and at least 20 Dinka civilians, and there have been reports of ethnically motivated killings and attacks in the capital Juba and elsewhere.

Foreign governments, including Britain, Kenya, Uganda and the United States, have been evacuating their nationals. On Saturday four US servicemen were wounded when their aircraft came under fire in a rebel-held area.

Oil production accounts for more than 95 per cent of South Sudan's fledgling economy, and the sector has been hit with oil companies evacuating employees after the death of at least five South Sudanese oil workers last week.