South Sudan faces challenges of "huge dimensions" as the world's newest nation struggles to support hundreds of thousands of people returning home or fleeing violence, the UN refugee chief said Tuesday.

"South Sudan is a newborn country, with six months as an independent sovereign state, but it is facing a multiplicity of humanitarian challenges of huge dimensions," said Antonio Guterres, the UN High Commissioner for Refugees.

The fledgling nation, one of world's least developed countries, needs "massive solidarity from the international community" to cope, Guterres told reporters, as he warned of another potential looming crisis of further arrivals.

Over 300,000 people are displaced by violence in the south, while the south also hosts some 80,000 refugees from the north and over 100,000 displaced people from the border region of Abyei, which both sides claim ownership of.

In addition, over 350,000 South Sudanese have returned to their homeland since October 2010 from the north, where they fled to during the war.

But an estimated 700,000 southerners remain in north Sudan, where aid officials are increasingly concerned for their future, with an April deadline approaching for them to either register or leave Sudan.

"If you apply mathematics to this situation... it is absolutely impossible to bring 700,000 people in a humane and dignified way" by April, Guterres said, adding he believed the majority wanted to return to the south.

"It is a huge undertaking, and it needs a very close and constructive cooperation of the two governments," he said, adding that after talks with southern leaders, he was due to travel to the capital Khartoum later on Tuesday.

"I'm here on this mission to appeal to the two governments, in order to ensure that they come together" to respect people's rights, he said.

South Sudan separated from Sudan in July after an overwhelming vote for independence that followed more than two decades of civil war.