The South Sudan Liberation Army said Saturday it was headed to Warrap state after an earlier attack on Mayom town in neighboring Unity state.
Both states are in South Sudan, near the border with Sudan.
The goal is to "liberate" the state from the government in Juba, the group said in a statement.
"Within few days, the people of Warrap will be liberated from abject poverty, corruption and abuse of human rights," the militia said. "We would also advise the civilians to evacuate all towns and move to villages in order to be safe," it said.
The warning follows an attack in oil-rich Unity state that killed 39 people, a South Sudan government spokesman said Saturday.
Liberation army militias carried out the attacks against civilians, spokesman Yein Matthew said.
South Sudanese government forces were pursuing the attackers through nearby woods, according to Matthew. One militia leader has been captured and is being questioned.
Liberation army members have clashed with the military of South Sudan, which separated from Sudan and became independent in July. Led by former officers of the southern army that fought neighboring Sudan in a 22-year civil war, the militias have taken up arms against their former comrades and become a challenge for the world's newest nation.The Sout