South Sudanese rebels say they have seized the capital of oil-producing Unity state, Bentiu, and have warned oil firms to pack up and leave within a week.
Thousands of people have been killed and more than one million displaced since fighting erupted in South Sudan in the middle of December, prompted by a power struggle between President Salva Kiir and former Vice President Riek Machar.
Humanitarian aid groups say they are in a race against time to prevent famine.
"The recapturing of Bentiu marks the first phase of liberation of oil fields from [the] anti-democratic and genocidal forces of Kiir," Lul Ruai Koang, rebel spokesman, said in a statement quoted by the Reuters news agency.
Urging all oil firms operating in government-held areas to shut their operations and evacuate their staff within a week, he said: "Failure to comply with this request, the oil companies risk forced oil shutdown and the safety of their staff."
An Oil Ministry official told Reuters on condition of anonymity that three Russian oil workers were injured in a rebel attack at a newly built refinery facility in Bentiu on Tuesday.
Phillip Aguer, South Sudan army spokesman, said there was fighting in Unity state but he did not have a full report on what had happened.
"There has been serious fighting in Unity today, so far the SPLA [government] forces are still on the ground but we are still waiting for a full report to tell us exactly what has happened," he said.
The civil war in Africa's newest state has created a humanitarian crisis in the country, which declared independence from Sudan in 2011, but has since been plagued by disorder.
The conflict has also disrupted oil production, which provides a hefty portion of the government's revenue.
Upper Nile state is the only province pumping oil in South Sudan since the shutdown in Unity.
A ceasefire signed in January has been broken frequently and peace talks in Ethiopian capital Addis Ababa have stalled often.
The US, Britain, the EU and Norway have threatened measures against the country's rival sides.
Last month, US President Barack Obama authorised possible targeted sanctions against those committing human rights abuses or undermining democracy and obstructing the peace process.