Juba - The first UN peacekeeping reinforcements arrived on Friday in South Sudan, where the government is said to have agreed an immediate ceasefire after nearly two weeks of heavy fighting with rebels.
The United Nations warned that tensions remained dangerously high despite efforts to halt a slide into civil war in the world's youngest nation which is believed to have left thousands dead.
East African leaders acting as peace brokers announced on Friday that the government of South Sudanese President Salva Kiir had agreed to a ceasefire.
But the de facto leader of the rebels, Riek Machar - whom Kiir accuses of having tried to mount a coup after being sacked as vice president in July - would not immediately commit to a truce.
In a satellite telephone interview with the BBC from an undisclosed located, Machar said a mechanism was required to monitor any ceasefire.
"For the ceasefire to be credible there is need for a mechanism, or else we will be deceiving ourselves," he said.
He also demanded that Kiir release all 11 of his political allies who were arrested right at the beginning of the unrest, while acknowledging that two of them had been freed.
The regional leaders brokering the end to hostilities have given Machar and Kiir four days to hold face-to-face talks and halt fighting, pledging unspecified "further action" if the civil war continued.
Meanwhile the UN said dangers remained so acute that large numbers of bodies had been seen outside at least one UN base, but could not be collected.
A 72-member UN police contingent arrived in the country on Friday, according to a UN spokesperson in New York. They are the spearhead of what is to be 6 000 extra troops authorised by the UN Security Council to bolster the hard pressed UN mission.
More troops and equipment were expected to arrive on Saturday.
Once all the reinforcements are in, they 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.
That mission has so far been badly stretched as fighting has claimed thousands of lives since erupting 15 December.
The UN says more than 120 000 people have fled their homes, including 63 000 sheltering in UN peacekeeping bases, and neighbouring nations and world powers feared South Sudan was sliding into civil war.
Fighting in oil-rich zones
Since it started, the fighting has spread to half of South Sudan's 10 states, with the violence taking on an ethnic dimension, pitting members of Kiir's Dinka tribe against Machar's Nuer community and atrocities reported to have been carried out by both sides.
The warring forces are locked in fierce battles for control of several strategic oil-producing areas in the north of the country, which only won independence from Sudan in 2011.
Witnesses have reported heavy clashes in Malakal, capital of the oil-producing Upper Nile State, which both sides claimed to control.
A rebel spokesperson in the area, Moses Ruai Lat, told AFP that "the whole of Malakal" was now in the hands of Machar's loyalists.
But South Sudanese Defence Minister Kuol Manyang Juuk dismissed that claim as "disinformation" and asserted that Machar's loyalists "are no longer in Malakal - the town is under full government control".
Rival sides have also massed their forces around Bentiu, a key city in the oil-producing state of Unity as well as Bor, the capital of Jonglei state.
"Government forces are believed to have consolidated their positions in and around Bor," said a UN statement. "Anti-government forces remain in the vicinity and the situation remains tense."
UN aid workers only returned to Bor on Thursday to help displaced civilians, said the UN's Office for the Coordination for Humanitarian Affairs (OCHA).
"Following the intensive fighting for the town, there are reportedly a large number of bodies in the open near the base, which poses a risk for disease outbreaks," the office added.
The United Nations has rushed "critical assets" to the country, where it says it is struggling to cope with the dual role of protecting as well as feeding and sheltering terrified civilians.
Crude prices have edged higher because of the fighting as oil production, which accounts for more than 95% of South Sudan's economy, was dented by the violence and oil workers evacuated.