By Denis Dumo
JUBA (Reuters) - South Sudanese rebels killed "many" government soldiers in three days of fighting in Malakal, the capital of oil-producing Upper Nile State, a rebel military spokesman said on Monday.
The rebel forces brought down a government helicopter gunship that had been sent to attack rebel positions in the town on Sunday, Lony Ngundeng told Reuters.
"The government forces have lost many soldiers," he said.
Government spokesmen were not immediately available to comment.
The world's newest state, which declared independence from Sudan in 2011, was plunged into conflict nearly 18 months ago between forces loyal to President Salva Kiir and rebels allied with his former deputy, Riek Machar.
Several ceasefires have been agreed and broken and each side accuses the other of violating one announced in early February.
The conflict reopened ethnic fault lines between Kiir's Dinka people and Machar's forces, who are largely ethnic Nuer.
IGAD, the regional group that acts as the mediator in the crisis, said the rebels had ganged up with the Shilluk militia of General Johnson Olony to attack the government's positions in and around Malakal.
It was not immediately clear if Olony, a major general in the government's army, had officially defected to the rebel side. Ngundeng said the rebels and Olony's forces jointly controlled the town.
"The IGAD Mediation is deeply frustrated by the spread of violence to Upper Nile and strongly condemns this serious violation of the Cessation of Hostilities Agreement," the group said in a statement. It urged the warring parties to end the killings.
Malakal is the operations hub for aid agencies in the state, where an estimated 245,000 people have been displaced by fighting.
The United Nations Security Council also condemned the renewed fighting.
"The members of the Security Council condemned in the strongest terms the repeated violations of the Cessation of Hostilities Agreement accepted and signed by the Republic of South Sudan and the SPLM/A (in Opposition) on 23 January 2014," it said in a statement on Sunday.