South Sudanese Labour and Public Service minister Lieutenant-General Gabriel Duop Lam has resigned.
Lt-Gen Lam cited inconsistency in President Salva Kiir's leadership as the reason for his resignation.
He is a member of the opposition faction headed by First Vice President Taban Deng Gai.
Lt-Gen Lam said he was unhappy with the direction President Kiir was leading the country.
He described the Unity Government as partial transitional government of national unity.
“Without wasting much of your time, ladies and gentlemen of the press, I Lt. General Gabriel Duop Lam, the National Minister of Labour, Public Service and Human Resource Development in the Republic of South Sudan, with sound mind and excellent rule of reasoning, do hereby officially resign from the position of being the minister in the so called Government of National Unity with effect from February 17,” he said in letter circulated to the media in Juba.
Lt-Gen Duop also declared allegiance to the rebel leader, Dr Riek Machar, who is leading an armed struggle against President Kiir.
Last week, a top army commander, Gen Thomas Cirilo, also cut ties with President Kiir and quit military service.
South Sudan peace remains at stake despite pledges from the president to enforce stability across the country.
Over a million South Sudanese have become refugees since the conflict erupted in 2013. Thousands more have lost their lives.
The security situation remains tense in Eastern and Central Equatoria and in Upper Nile’s Malakal and Unity areas.