President Salva Kiir's spokesperson Ateny Wek Ateny told Xinhua that last week's visit to Khartoum by Malong violates the agreement the latter signed with the government in November last year upon his release from house arrest.

"His (Malong) going to Khartoum is not right," Ateny said in Juba. "No one has given him permission, and for us it seems he is looking toward subversion."

The former army chief was released from house arrest in November 2017 and allowed to seek medical attention in Kenya.

The government declared him a rebel in January after obtaining leaked audio clips of the general commanding his loyalists to attack government positions.

"Going to Khartoum is not part of what was agreed to be observed. He (Malong) also gave his word that he will not do what is seen as subversion," Ateny said.

Malong and three other senior military officers, including those from the armed opposition (SPLA-IO), are on the U.S. Department of the Treasury's list of people subject to travel ban and asset freeze for stalling peace efforts in South Sudan.

South Sudan descended into violence in December 2013 after President Kiir sacked his former deputy Riek Machar, leading to fighting between mostly Dinka ethnic soldiers loyal to Kiir and ethnic Nuer soldiers loyal to Machar.

The violence has led to the death of tens of thousands and forced four million others to flee to neighboring countries.

A 2015 peace agreement collapsed following renewed violence in July 2016. Fresh peace efforts have been stalled due to lack of progress.