South Sudan's army is advancing on a key rebel enclave, a spokesman said Monday, as UN and aid agencies warned fighting had cut hundreds of thousands of civilians off from "life-saving aid".

The government assault that began late April is one of the heaviest offensives in the 17-month long civil war and has cut off over 650,000 from aid, with gunmen raping, torching towns and looting relief supplies, according to the UN and aid agencies.

Government forces have been pushing south from the government-held town of Bentiu, state capital of Unity state, towards the opposition zone around the town of Leer.

"We have pursued the rebels from around Bentiu up to Leer," South Sudan army spokesman Philip Aguer said.

It was not immediatly clear if fighting had reached the town itself, but the UN warned of its devastating impact in both Unity and Upper Nile, where rebels attacked the state capital Malakal on Friday.

"UN agencies and their partners are working to address the immense humanitarian consequences of the violence, which has resulted in more than 650,000 civilians being left without life-saving aid," UN aid chief in South Sudan, Toby Lanzer, said in a statement.

Fighting broke out in December 2013 when President Salva Kiir accused his former deputy Riek Machar of attempting a coup, setting off a cycle of retaliatory killings across the country.

- 'Unimaginably difficult conditions' -

Leer, the birthplace of Machar, was ransacked by government forces in January 2014, with gunmen looting and torching the hospital there run by Doctors Without Borders (MSF).

MSF has since rebuilt the hospital, the only referral facility in opposition areas.

"Military operations in Unity and Upper Nile states over the past three days in particular have again devastated countless lives," Lanzer said.

"Eyewitness accounts report targeted rape and killing of civilians, including children. The offensive in Unity has left thousands of homes burnt and Leer hospital is again under threat of destruction," he added.

The UN children's agency said girls as young as seven had been raped or killed, boys as young as 10 had been killed and others had been mutilated or abducted by "armed groups aligned with" the army.

"The deliberate targeting of children in these attacks is an outrage," UNICEF country head Jonathan Veitch said.

The International Committee of the Red Cross (ICRC), forced to pull out of Leer, said it was deeply worried for the fate of those left without food or healthcare, as the fighting comes closer.

"We fear that the situation of some 100,000 people in Leer, who are now hiding in unimaginably difficult conditions, will worsen day by day," said ICRC chief in South Sudan, Franz Rauchenstein, adding that "prolonged displacement exposes people to suffering".

The army said it was still battling for control of Malakal, gateway to the country's last remaining major oil fields, after a pro-government general there swapped sides to join the rebels, taking his tribal militia with him.

Officials said the attack began just before darkness on Friday, with rebels crossing the White Nile river on boats aided by militia commander Johnson Olony.

The town has swapped hands multiple times in the war, and much of Malakal was already in ruins.

"Fighting is ongoing," Aguer said. "Olony has finally declared himself as part of Riek Machar's force."

Olony spokesman Nyagwal Ajak Denk said the force -- an ethnic Shilluk militia accused of abducting children -- was in "full control of Malakal town".

Rebel spokesman Mabior Garang confirmed that Olony was now an ally, saying their interests "currently coincide", although he had not formally joined with Machar's troops.

Over half of the country's 12 million people are in need of aid, with 2.5 million people facing severe food insecurity, according to the UN.

Analysts and aid workers believe tens of thousands of people have been killed in the fighting in the world's youngest nation, which only gained independence from Sudan in 2011.