South Sudan President Salva Kiir has accused the UN peacekeeping mission of acting like a “parallel government” in his country.
His comments come after his government accused the UN of hiding rebels and guns at their camps.
More than 70,000 civilians are seeking shelter at UN bases across South Sudan after fighting broke out last month.
The UN says both government soldiers and rebels have committed atrocities in one of the world’s poorest countries.
What started out as a political dispute between Mr Kiir and his former deputy Riek Machar on 15 December has escalated into full-scale conflict, with reports of ethnic killings.
Around 500,000 people have been displaced and the UN estimates that considerably more than 1,000 have been killed.
The UN is in the process of deploying an extra 5,500 peacekeepers to South Sudan, to bring its forces up to 12,500.
Mr Kiir made his comments, broadcast on national TV, following the weekend capture of Bor from rebel forces.
The South Sudanese army also says it has recaptured the key town of Malakal after days of heavy fighting, though this is disputed by the rebels.