Parts of war-ravaged South Sudan have been hit by famine, according to a government official, with nearly half the country's population lacking reliable access to affordable food by July.

Oil-rich South Sudan has been mired in civil war since 2013, when President Salva Kii fired his deputy.

Since then the fighting has increasingly split the country along ethnic lines, leading the United Nations to warn of a potential genocide.

The fighting has prevented many farmers from harvesting their crops while hyper inflation, which reached more than 800% last year, has put the price of imported food beyond the reach of many.

Parts of the country have also been hit by drought.

In greater Unity (state), some counties are classified in famine or ... risk of famine, Isaiah Chol Aruai, chairman of South Sudans National Bureau of Statistic, told a news conference in Juba.

Mr Aruai said the impact of the war, combined with high food prices, economic disruption and low agricultural production was expected to make 4.9 million people what is termed "food insecure" between February and April, with that number rising to 5.5 million by July.

"Famine has become a tragic reality in parts of South Sudan and our worst fears have been realised," Food and Agriculture Organisation South Sudan representative, Serge Tissot, said at the same news conference.

Trocaire warn of dwindling food supplies

The Director of Trocaire's International Division has warned that tens of millions of people in South Sudan and Somalia are facing a "really, really difficult few months."

Sean Farrell, who is just back from South Sudan, said food supplies are practically gone and they were seeing levels of hunger now, when the next crop season in South Sudan is not for another six months.

Speaking on RTÉ's News at One, he said those who were not on food aid were resorting to eating wild fruits and berries.

Mr Farrell said while people are "not dying yet", it was the start of the crisis and people will die, if action is not taken.

He said there is an opportunity to prevent a lot of deaths and called on the international community to step up and answer Trocaire's calls for aid.

According to the United Nations, famine is declared when at least 20% of households in an area face extreme food shortages, acute malnutrition rates exceed 30%, and two or more people per 10,000 are dying per day.

The fighting has uprooted more than three million people and a UN report released today said continuing displacement presented "heightened risks of prolonged (food) under production into 2018".

Many parts of the country are very hard to reach. Six years after independence from neighbouring Sudan, South Sudan only has only 200km of paved roads in a nation the size of Texas. Fighting also impedes aid delivery; warehouses have been looted and aid workers have been killed.

This month, in a sign the war was taking a turn for the worse, Mr Kiir's government has been hit by high-profile defections.

Two top military officials resigned their positions, citing ethnic favouritism, human rights abuses and others charges.

Punishments handed out to some soldiers from the Dinka, Mr Kiir's tribe, for crimes including rape and murder were being set aside, Colonel Khalid Ono Loki, one of officials who resigned said.

The minister of labour has also defected to the rebels.