JUBA, South Sudan — Fighting resumed in South Sudan after rebel forces attacked the capital of the oil-producing state of Upper Nile, a military official said Tuesday, in what appeared to be the heaviest clashes since both sides signed a cease-fire last month.
Fighting broke out early Tuesday in Malakal, which once was in rebel hands but is now controlled by government troops, said South Sudanese military spokesman Colonel Philip Aguer.
‘‘We were expecting it because the rebel commander in the area said he would not respect a cease-fire,’’ he said.
Although the country’s warring factions signed a cease-fire on Jan. 23, both sides have repeatedly accused each other of violating the agreement. The international community has repeatedly urged both sides to respect the truce as peace talks continue in neighboring Ethiopia. Those talks have proceeded slowly.
Toby Lanzer, the United Nations’ top humanitarian official in South Sudan, said on Twitter Tuesday that all groups ‘‘engaged in the violence must uphold people’s rights and protect noncombatants.’’ The UN says both sides have committed rights violations, and South Sudan’s military announced Monday that more than 20 government soldiers had been charged in civilian killings.
Grace Cahill, a spokeswoman for Oxfam in South Sudan, said armed groups had gathered outside the UN compound in Malakal, where 27,000 people have been seeking shelter.
‘‘The presence of armed groups outside the compound has made those inside very scared,’’ she said.
Thousands of people have been killed and more than 800,000 displaced by violence since mid-December, when a fight broke out among presidential guards in the capital, Juba, before spreading out across the country. Ugandan forces are fighting alongside the South Sudanese military as it tries to put down a rebellion led by former vice president Riek Machar, an influential politician whose dismissal last year sparked ethnic tension in a country with a history of divided military loyalties.
Ugandan Foreign Minister Sam Kutesa said Tuesday that Uganda will not withdraw its troops from South Sudan until the African Union deploys a planned ‘‘standby force’’ in the country. The African Union says such a force would have the capacity to respond rapidly to outbreaks of violence across the continent, but it may take months — even years — to realize the plan.
The United States has urged all foreign militaries fighting in South Sudan to withdraw from the country, and the rebels have said the presence of Ugandan troops is an obstacle to achieving a peace deal.
Kutesa said an abrupt withdrawal of Ugandan troops from South Sudan would leave ‘‘a security vacuum that can be taken advantage of.’’
The United Nations says both sides in South Sudan’s conflict have committed rights violations in fighting that was often ethnically charged.
Machar is a member of the Nuer ethnic group, as are most of the soldiers who defected and joined his rebellion late last year. Most of the loyalist forces are from the Dinka ethnic group of President Salva Kiir, whose government insists that the unrest was sparked by a failed military coup mounted by soldiers loyal to Machar.
Machar denies the coup allegation but says his goal is to have Kiir removed from power.