At least 56 rebels and four government soldiers have been killed in heavy clashes in South Sudan, in a worrying new surge of violence.

A Sudan People’s Liberation Army (SPLA) spokesman, Brig Gen Lul Ruai Koang, said that rebels aligned with the former vice-president, Riek Machar, attacked government troops near the country’s second-largest city of Malakal.

Violence between Machar’s supporters and troops loyal to his political foe, President Salva Kiir, has blighted the nation for much of its hard-won independence from Sudan in 2011.

“The rebels of Riek Machar attacked us in two places of our defence, that is Wajwok and Lelo, and we heavily impacted on them. The dead bodies of the rebels confirmed after [the] count was 56,” the spokesman said, adding that the clashes began on Friday evening and ended the next day.

“On our side we lost four SPLA fighters and 20 were wounded.”

Malakal is the closest town to the oilfields in the Upper Nile region that bring in the government’s greatest revenue, and has changed hands several times since fighting broke out in 2013.

Forces loyal to Kiir invited journalists to the region on Sunday to show they were in control of the area and a photographer reported seeing up to 40 bodies.

However, Maj Dickson Gatluak, with the rebel SPLA in Opposition (SPLA-IO), insisted his forces were in control of two towns as well as the northern part of Upper Nile state. “What we are trying to do now is, our forces are trying our level best to try to march to Malakal town. So our forces are not yet displaced from those areas and this is not true,” he said.

Gatluak said fighting was continuing, although an AFP photographer reported it was calm on Sunday.

“It is just a matter of time [before] our forces will take Malakal ... we have realised that there is no political settlement in Juba, there is not any political space in Juba, so we have to organise ourselves because the government has failed,” he said.

South Sudan descended into war just two and a half years after it gained independence, when Kiir accused Machar – whom he had sacked as his deputy – of plotting a coup in December 2013.

Kiir is a member of the Dinka tribe, while Machar is a Nuer and fighting has split the country along ethnic lines.

Numerous attempts to shore up a fragile 2015 truce have failed and, in a major setback to peace efforts, fierce clashes erupted in Juba on 8 July this year between Kiir’s forces and those loyal to Machar.

The international community has expressed deep concern over a spread in violence since the July clashes, which pushed the number of refugees past a million, according to the UN.

In a further blow to peace hopes, Machar urged “a popular armed resistance” against his rival’s government last month.

Machar, who fled to Khartoum after the July fighting, left for South Africa last week for medical tests.

Last week an influential group of South Sudanese politicians known as the “former detainees” – after their arrest when war broke out in 2013 – warned that “the country is on the edge of a precipice”.

In a sign of growing frustration in the region with the warring leaders of South Sudan, Kenyan politicians threatened sanctions last week. The defence and foreign relations committee chairman, Ndung’u Gethenji, said they were considering sanctions “against those who are perpetuating war-like activities, genocide and killing [so that they] may no longer enjoy having their assets in our country, their children studying in our schools in relative comfort – in peace and security – while they continue to destroy the lives of their people back in their own country.

“Kenya can no longer afford to have another failed state on its border,” he said.