NAIROBI (Reuters) - South Sudan's rebels said a government offensive was pushing toward their main stronghold in the northeast on Friday, as the United Nations announced at least 25 aid workers had been evacuated from the town amid escalating clashes.
There were growing fears for civilians caught up in fighting, the U.N.'s Office for Coordination of Humanitarian Affairs said.
The loss of Pagak in the northeastern Upper Nile region would be a major blow to the cash-strapped rebels, whose leader Riek Machar is being held under house arrest in South Africa.
Pagak has been the main rebel base since 2014. It is located deep in the heartland of the Nuer people, pressed against the Ethiopian border.
South Sudan's civil war began in December 2013, when President Salva Kiir, an ethnic Dinka, fired his deputy, Machar, a Nuer. Since then, the conflict has displaced a quarter of the 12 million-strong population and plunged parts of the country into famine briefly earlier this year.
The U.N. has said the ethnic violence in the oil-rich nation could set the stage for genocide. Rights groups say soldiers and allied militias have raped, tortured and killed civilians. The rebels have also been accused of major human rights violations.
"The government launched an attack towards Pagak for about a week now," rebel spokesman Lam Paul Gabriel told Reuters via Whatsapp message.
Gabriel said "many" civilians lost their lives while others fled to swamps or crossed to Ethiopia.
But a military spokesman denied there was any fighting because the government was observing a unilateral ceasefire.
"There is nothing like that so far, we had a briefing this morning and there is no fighting at Pagak," said Colonel Santo Domic Chol. "Pagak is the headquarters of the IO (rebels) where they have their ammunition and everything and of course it is not easy for SPLA (the military) to access such the area unless it had gone for major operations."
The military is known as the Sudan People's Liberation Army (SPLA) and the rebels as the Sudan People's Liberation Army-In Opposition (SPLA-IO).
At least 25 aid workers were evacuated from Pagak on Thursday and Friday, and fighting in the nearby Mathiang area forced thousands of civilians to flee, the U.N.'s Office for Coordination of Humanitarian Affairs said in a statement.
OCHA said the fighting began last week.
"Humanitarians are deeply concerned regarding the plight of civilians in areas affected by the clashes," the statement read.
An internationally-backed peace deal between Kiir and Machar collapsed last year. Since then, the government has made steady gains across the Upper Nile region and the rebels have shifted their focus to the country's south and taken control of areas there.
(Editing by Andrew Heavens)