The South Sudanese government says it has lost control of the capital of a key oil-producing state to fighters loyal to rebel leader Riek Machar.
Colonel Philip Aguer, the South Sudanese military spokesman, said on Sunday that Bentiu, the capital of Unity state, is now under the control of rebels.
"Bentiu is in the hands of a commander who has declared support for Machar," he said. "Bentiu is not in our hands."
Machar, the former vice president, said on Saturday that his troops have captured the state of Unity and have control of much of the country.
The political crisis began a week ago, after President Salva Kiir accused Machar of attempting a coup. Reports say at least 500 people have been killed in the violence since then.
The fighting between troops loyal to Kiir, who is from the Dinka ethnic group, and opposition leader Machar, a Nuer, erupted around Juba on December 15.
The South Sudanese president accused his archrival, Machar, of attempting to topple his government, but he said the coup attempt had been foiled.
The government said on Tuesday that ten senior political figures had been arrested after the alleged coup attempt.
Machar has denied the accusation that he had led a coup attempt.
South Sudanese Information Minister Michael Makuei Lueth said Machar was believed to be hiding in Unity state.
"He is a rebel, he's a renegade and we are looking for him. He's moving in the bushes of South Sudan," Lueth said of Machar.
US President Barack Obama said on Sunday that Washington would take action if needed after US forces were attacked in South Sudan.
"As I monitor the situation in South Sudan, I may take further action to support the security of US citizens, personnel and property, including our embassy, in South Sudan," Obama wrote in a letter to Congress.
He made the remarks after a US military aircraft came under attack in the violence-hit country, injuring four American soldiers, one of them critically.
The aircraft came under attack during a reported evacuation mission in Bor city, the capital of the state of Jonglei.
South Sudan gained independence in July 2011 after its people overwhelmingly voted in a referendum for a split from the North.
The government in Juba is grappling with rampant corruption, unrest and conflict in the deeply impoverished but oil-rich nation, left devastated by decades of war.