JUBA (AFP) - International pressure bore down on Saturday, Dec 28, 2013, on the two sides in South Sudan's bloody violence to open peace talks to keep the young nation from sliding into civil war.

East African and Horn of Africa peace brokers gave until December 31 for President Salva Kiir and de facto rebel leader Riek Machar, whom Mr Kiir sacked as vice-president in July, to start face-to-face talks and stop two weeks of fighting that is thought to have left thousands dead.

"We, the government, are ready to meet even before that," South Sudan's Vice-President James Wani Igga told reporters. "It's now up to Machar to accept the ceasefire." The government on Saturday reiterated accusations that Mr Machar was mobilising thousands of youths to attack its interests.

"Dr Riek mobilises his... youths, up to 25,000... and wants to use them to attack the government" in the eastern state of Jonglei, where rebels said to support Mr Machar briefly captured the regional capital Bor earlier this month, government spokesman Michael Makuei told Agence France-Presse.

"They are able to attack any time," he added. "We are in a state of alert to protect the civilian populations." But rebels spokesman Moses Ruai Lat rejected this, saying the former vice-president was "not mobilising his tribe," the Nuer, South Sudan's second biggest ethnic group.

Those young people were regular soldiers turning their back on the government and had not been drafted by Mr Machar, he added.

The regional grouping the Inter-Governmental Authority on Development (IGAD) is spearheading efforts to end the fierce battles for control over several strategic oil-producing areas notably in the north of South Sudan.

The United Nations, Washington and Beijing are also pressing for talks.

"IGAD has already come out with the condition that the contending parties should negotiate within four days beginning from Friday," Ethiopian foreign ministry spokesman Dina Mufti said on Saturday. "So we are awaiting results."

Spokesmen for IGAD said President Kiir had on Friday expressed willingness for an "immediate" ceasefire though Mr Machar would not immediately commit to a truce.

The rebel leader said he first wanted a mechanism to monitor any ceasefire as well as the release of all his political allies arrested when trouble first broke out.

The conflict, fuelled by an old rivalry between Mr Kiir and Mr Machar, has fanned ethnic differences between Mr Kiir's Dinka group and Mr Machar's Nuer clan in the country, which won independence from Sudan in 2011.

Reports of massacres, rapes and murders have emerged in recent days. The United Nations - whose hard-pressed peacekeepers are to be doubled to more than 12,000 - said one mass grave had been discovered and large numbers of uncollected bodies were seen outside at least one UN base.

The fighting erupted on December 15 after Mr Kiir accused his former vice-president of trying to mount a coup. Mr Machar has denied the allegation and retorted that Mr Kiir was trying to eliminate his rivals.

Rebels quickly took control of a few key regional cities including Bentiu, in the northern oil-producing state of Unity, as well as Bor which was recaptured by the army on Tuesday.

On Saturday, South Sudan army spokesman Philip Aguer told Agence France-Presse that all was calm in the main flashpoint areas of recent days, including Malakal, capital of the oil-rich north-eastern Upper Nile state, in Bor and in Unity state where the army said it pushed back rebel attacks on Friday.

Mr Aguer insisted his troops would implement any truce agreed by the government and rebels but vowed they would riposte if attacked.

The death toll nationwide is said to be several thousand. The United Nations has said more than 120,000 residents have been displaced since the conflict started.

A source at the UN High Commissioner for Refugees in the Sudanese capital Khartoum told Agence France-Presse that the agency was investigating reports that hundreds of South Sudanese had fled into neighbouring Sudan.

UN Secretary General Ban Ki Moon this week condemned the ethnic attacks as "a grave violation of human rights" and warned that those responsible would be "held accountable".

The United Nations sent in the first peacekeeping reinforcements on Friday - a 72-member UN police unit from Bangladesh - and more troops and equipment were expected to arrive on Saturday.

The extra troops will almost double the size of the UNMISS mission in the country to a total of up to 12,500 soldiers and 1,300 police.

South Sudan, the world's youngest nation, became independent after a civil war that killed more than two million people between 1983 and 2005.