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 brief lull in hostilities in the country's 10-month-old civil war after the end of the rainy season which made many roads impassable.

"Bentiu is under attack from the south and north," army spokesman Philip Aguer said.

An aid worker in the United Nations base outside the town reported explosions throughout Wednesday afternoon, but it was not possible to verify who was in control.

The town, state capital of the previously key oil-producing Unity state, has changed hands several times since the war broke out in December 2013, but has been in government hands since May.

When rebels loyal to ousted vice president Riek Machar stormed the town in April, they unleashed two days of ethnic slaughter as they hunted down civilians sheltering in mosques, churches and a hospital, according to the UN.

Both sides in the conflict -- Machar's forces and troops loyal to President Salva Kiir -- have been accused of war crimes including mass killings, rape, attacks on hospitals and places of worship and recruiting child soldiers.

Thousands of people have been killed and almost two million have been forced from their homes, including almost 100,000 people who are sheltering in squalid UN peacekeeping bases -- including in Bentiu -- fearing they will be killed if they leave.

The world's newest country is blighted by what UN officials call "a man-made crisis" that has caused widespread hunger and left a third of the population needing help.

The International Crisis Group on Wednesday warned both sides were "preparing for major offensives" that would likely result in "widespread displacement, atrocity crimes and famine."

Both sides have spent the recent few months of relative calm to prepare for battle, using the time to "import arms and marshal forces that were hastily mobilised at the outset of war in December," the ICG report added.

While long-running peace talks in Ethiopia are currently stalled, Kiir and his sacked vice-president Machar met earlier this month in Tanzania, shaking hands and accepting mutual responsibility for the war.

It was their first meeting since signing a ceasefire in August, which like three previous agreements, swiftly collapsed.