An earth bank, topped in some places with a coil of razor wire, surrounds the United Nations peacekeeping base outside Bentiu in South Sudan. There are also watchtowers and armed peacekeepers.

Inside the 6.5 kilometer (four mile) perimeter fortifications live 118,000 people, some of the 2.2 million uprooted by civil war since December 2013.

Washington on Thursday slammed fresh fighting in South Sudan, and denounced the "senseless man-made conflict" that has left thousands dead and forced millions to flee their homes.

"The United States condemns in the strongest terms the latest attacks" by rebels in and around Bentiu, State Department spokeswoman Jen Psaki said in a sharply-worded statement.

Fierce fighting has erupted in South Sudan's key northern oil hub of Bentiu, with rebels on the attack to recapture the town from government forces, the army and aid workers said Wednesday.

The fighting also marks an end to a slight lull in hostilities in the country's 10-month-old civil war following the end of the rainy season which made many roads impassable.

Warring forces in South Sudan traded blame Wednesday for the crash of a U.N. helicopter in which three crew died, although investigations have yet to confirm if it was shot down.

South Sudanese troops and rebels battled in the key oil town of Bentiu Monday as the government waged a major offensive, despite insisting peace talks would go ahead to end the four-month long civil war.

Army spokesman Philip Aguer said there had been heavy fighting Monday in the northern town, state capital of the oil-producing Unity state, a day after government troops moved to wrest back control.

South Sudan's army said on Tuesday it was battling rebels in three key states, as insurgents blamed for massacring hundreds of civilians in attacks last week continued an offensive targeting oil fields.

The rebels seized the town of Bentiu last week, unleashing two days of ethnic slaughter as they hunted down civilians sheltering in mosques, churches and a hospital, according to the U.N.

Rebels in South Sudan battled government forces Thursday in a renewed offensive targeting the young nation's key oil fields, the army said, amid famine warnings by the United Nations.

Fighters loyal to rebel chief Riek Machar recaptured the town of Bentiu on Tuesday, the state capital of oil-producing Unity, with U.N. peacekeepers reporting corpses littering the streets.

South Sudan's army said Wednesday rebels had wrested control of the key oil town of Bentiu, one of the most bitterly contested regions in the four-month-long conflict.

The town, capital of oil-producing Unity state, is the first major settlement to have been retaken in a renewed offensive by forces of rebel leader Riek Machar, a former vice president.

The leader of South Sudan's rebels has vowed to attack the capital Juba and target crucial oil fields, warning in an exclusive interview with AFP that the civil war will not end until the country's president is removed from power.

Former vice president turned rebel chief Riek Machar branded his arch rival, President Salva Kiir a "dictator" and said he saw "no reason for power sharing".

Sudanese war planes on Monday launched a fresh bombing raid on Bentiu, capital of the oil-rich South Sudan border state of Unity, killing at least one child, officials said.

"This is a serious escalation, and a violation of the territory of South Sudan... I think it is a clear provocation," Mac Paul, the South's deputy director of military intelligence said.