A spokesman for the vice-president has confirmed that South Sudan is "back to war", as rival factions clash and hundreds have been reported killed.
Forces loyal to Vice-President Riek Machar say government troops attacked their positions in the capital, Juba.
However, Information Minister Michael Makuei Lueth said the reports of war were "dishonest".
The UN mission said hundreds had sought shelter in its compounds, and called for leaders to restrain their troops.
UN Secretary-General Ban Ki-moon condemned what he called the "senseless violence". The UN Security Council is holding an emergency session to discuss South Sudan.