South Sudan's army admitted on Monday that it had lost control of Bantiu, capital of the oil- producing Unity State, media reported.
"Our forces have not prepared to control Bantiu," South Sudan army spokesman Philip Aguer was quoted by South Sudanese media as saying.
He said Bantiu was still under the control of the opposition forces, but the army was about to launch a military action to reclaim it, reported Xinhua.
Clashes are still continuing between South Sudan army and a military group which is loyal to former Vice-President Riek Machar.
Machar said Monday he was ready for dialogues with South Sudan President Salva Kiir Mayardit on condition that Kiir agrees to release the political detainees.
Meanwhile, the Sudan Tribune website reported that Kiir met Rebecca Garang, widow of the late historical leader of the ruling Sudan People's Liberation Movement (SPLM) John Grang.
Rebecca, who descends from the Dinka tribe, similarly as Kiir, said she discussed the president ib the situations in the south, noting that the dialogue would continue.
The clashes erupted in South Sudan last Sunday between two military factions, one descending from the Dinka tribe to which Kiir belongs, and the other descending from the Nuer tribe loyal to Machar, who is accused of plotting to topple the government.
Over 500 people have been killed since the eruption of the clashes which extended to cover many areas in the newly-born state, media reported.