South Sudan's brutal war rages on

Gunmen in South Sudan have raped girls, seized boys to become soldiers and torched towns in some of the heaviest fighting seen in the 17-month-long civil war, the United Nations said Tuesday.

Over 300,000 civilians have been left without "life-saving aid" in the northern battleground state of Unity, after the UN and aid agencies pulled out due to a surge in fighting, with over 100,000 forced to flee their homes.

The conflict has killed tens of thousands of people in the world's youngest nation

South Sudan's civil war began in December 2013 and has been characterised by ethnically-driven massacres, rape and attacks on civilians and medical facilities.

Peace talks in neighbouring Ethiopia have so far failed to reach any lasting agreement, or even an effective ceasefire.

The violence, which has escalated into an ethnic conflict involving multiple armed groups, has killed tens of thousands of people in the world's youngest nation, which gained independence from Sudan in 2011.

It has also left over half of the country's 12 million people in need of aid, with 2.5 million people facing severe food insecurity, according to the UN.

The UN peacekeeping mission said it was "increasingly concerned" about reports from Guit and Koch counties in Unity state of "towns and villages being burned, killings, abductions of males as young as 10 years of age, rape and abduction of girls and women, and the forced displacement of civilians."

The violence is some of the worst in months, as government forces push south from the state capital Bentiu into an opposition zone around the town of Leer, home to some of the country's once lucrative oil fields.

The International Committee of the Red Cross has also withdrawn staff from Leer and warned that escalating fighting between forces loyal to President Salva Kiir and rebel leader Riek Machar was forcing thousands of civilians to flee for their lives yet again.

Unity state governor Joseph Monytuil told reporters late Monday that government troops aimed to take Leer from opposition forces within days. "Our forces... are now pursuing them to where they came from," said Monytuil.

Doctors Without Borders (MSF) said Saturday it was forced to evacuate its foreign staff from Leer and halt all medical services amid fears the rebel-held town was about to come under "imminent attack" from government forces.

Leer, the birthplace of Machar, was ransacked by government forces in January 2014. Gunmen looted the MSF hospital and burned some of the buildings.

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

The European Union late Monday condemned the fighting, saying that "South Sudan's man-made crisis has caused one of the worst humanitarian disasters of recent years."

EU aid chief Christos Stylianides said there "can be no military solution to this conflict" and that responsibility for striking a peace deal rests on the shoulders of the leaders.

"If they fail to make the necessary effort for peace, they will inevitably be held responsible also for the consequences," he said in a statement.