South Sudan's army has captured a key rebel enclave in the northern battleground state of Unity, the information minister said Wednesday, after an almost month-long assault marred by accusations of rampant human rights abuses.

"Leer is fully controlled by the government -- in fact the whole of Unity State is now under the government," Information Minister Michael Makuei told AFP.

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 people from aid, with gunmen raping, torching towns and looting relief supplies, according to the United Nations and aid agencies.

Soldiers have been pushing south from the government-held town of Bentiu, state capital of Unity state, towards the town of Leer.

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.

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.

The UN children's agency this week said that in recent fighting 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 International Committee of the Red Cross (ICRC), forced to pull out of Leer earlier this month, said it was deeply worried for the fate of those left without food or healthcare.

UN aid chief in South Sudan, Toby Lanzer, on Monday said that "eyewitness accounts report targeted rape and killing of civilians, including children," and that the government offensive in Unity "has left thousands of homes burnt."

Rebels last week launched a major counter-attack, including an assault last week on Malakal, the state capital of Upper Nile and the gateway to the country's last remaining major oil fields.

The army is still battling for control of Malakal, Makeui said.

"For Malakal... the fighting is still going on in the suburbs of Malakal and we will soon know the final results," he said. Rebels claim to have captured the town.

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.