Updated

Thousands of South Sudanese have been displaced and aid workers evacuated as rebels on Saturday accused government troops of advancing on their stronghold and killing civilians along the way.

Opposition spokesman William Gatjiath Deng told The Associated Press that government troops have stepped up attacks in the past week in Mathiang in Longochuck county. The rebels have kept the troops from entering the rebel stronghold of Pagak but the fighting is getting worse, he said.

"The government is killing civilians, they're burning down houses," Deng said.

The reports of an offensive come as South Sudan prepares to mark its sixth anniversary of independence on Sunday.

South Sudan's military denied the rebels' accusations, saying there were no scheduled military operations in the area and suggesting that the opposition was fighting among themselves. The government earlier this year declared a unilateral cease-fire, but reports of fighting in several parts of the country have continued.

The United Nations said at least 25 aid workers have been evacuated from the Pagak area, which was established as the opposition headquarters when civil war began in late 2013. Aid workers in Pagak have said that if it falls to government forces, the rebels' position would be weakened.

The International Committee of the Red Cross said it has evacuated all staff and patients from Maiwut town and hospital. "We're concerned about the escalation of the conflict in the area and the humanitarian consequences of displacement and violence," said ICRC official Mari Mortvedt.

The U.N. also said about 50,000 civilians in the rebel-held area near the Ethiopian border have been cut off from humanitarian assistance.

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This version corrects to say Mathiang is in Longochuck county.