The 72 member police contingent on Friday, moved from Democratic Republic of Congo, were the first of up to 6,000 troops authorized by the UN Security Council to bolster the hard pressed UN mission.
The UN said dangers remain so acute that large numbers of bodies had been seen outside at least one UN base, but could not be collected.
UN peacekeeping spokesman Kieran Dwyer said the Bangladeshi police "will play a key role maintaining order and security" in UN bases where 63,000 people have sought refuge since fighting erupted on December 15.
More troops and equipment were expected to arrive on Saturday. More than 1,000 people have been killed since army forces loyal to President Salva Kiir and former vice president Riek Machar started fighting each other. The 15-nation Security Council voted on Tuesday to allow the reinforcements.
East African leaders said earlier that Kiir had agreed to a ceasefire. But Machar would not immediately commit to halting hostilities and the UN warned that both sides remain on a conflict footing.
The UN Mission in South Sudan (UNMISS) "says that the security situation in Upper Nile and Unity states is tense, with reports of the presence of anti-government and government forces," said a UN statement.
Diplomats said the rival sides were massing their forces around Malakal and Bentiu, the main cities in the two key oil producing states.
UNMISS says that at least 63,000 civilians are now sheltering in UN bases around the country, including 25,000 in two Juba compounds; 15,000 in Bor, 12,000 in Bentiu and 8,000 in Malakal.
Bentiu is "tense" and "there are reports that fighting may resume in the coming days," said the UN's Office for the Coordination for Humanitarian Affairs (OCHA) in a daily briefing report.
"Government forces are believed to have consolidated their positions in and around Bor," added the UN statement.

"Anti-government forces remain in the vicinity and the situation remains tense."
Bor is the capital of Jonglei state, a longstanding focus of the ethnic divisions highlighted by the new conflict. Machar is an ethnic Nuer, while South Sudan's president is from the rival Dinka tribe.


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