Clashes in Nasir County, Upper Nile State, between forces loyal to President Salva Kiir and First Vice President Riek Machar have threatened to undermine their fragile peace-sharing agreement.

"Our air force bombarded Nasir," information minister Michael Makuei Lueth told a press conference.

James Gatluak, the commissioner of Nasir County, estimated that 20 people were killed in the Sunday night attack. He claimed the airstrike was "directed to civilians".

Among the dead were "three children under five, two women, 14 teenage boys, and a sub-chief".

"One is currently in critical condition," Gatluak told AFP, calling on the South Sudanese government to embrace dialogue "instead of waging war against civilians".

Lueth said the strikes were part of "security operations", adding: "If you as a civilian happen to be there... then there is nothing we can do."

The fighting threatens a 2018 peace deal between Kiir and Machar, who fought a five-year civil war that killed some 400,000 people.

Kiir's allies have accused Machar's forces of fomenting unrest in Nasir County in league with the White Army, a loose band of armed youths from the vice-president's Nuer ethnic community.

Tensions spiked earlier this month when an estimated 6,000 White Army combatants overran a military encampment in Nasir.

An attempted rescue attempt by the United Nations led to the death of a UN helicopter pilot and senior South Sudanese general.

Lueth also confirmed the presence of Ugandan forces in Juba on a "military pact", a week after denying their deployment to South Sudan.

Last week Ugandan army chief Muhoozi Kainerugaba said Ugandan special forces "entered Juba to secure it".

The rising unrest has sparked international concern, with the United Nations Commission on Human Rights in South Sudan warning the country was seeing an "alarming regression" that threatened to undo years of progress.