Last week, Salva Kiir postponed presidential elections due next year to give government and rebel forces more time to achieve reconciliation.
It is estimated about a million people have fled their homes in South Sudan since unrest erupted last December after the president alleged his deputy, Riek Machar, was plotting a coup, which the latter has denied.
The fighting has pitted forces loyal to Mr Kiir’s government against rebels allied to Mr Machar, comprised of a loose coalition of armed civilians mobilised mainly on the basis of ethnic affiliations and defectors from the army.
The United Nations has accused both sides of crimes against humanity, including mass killings and gang-rape, and threatened sanctions against those responsible.
In a new interview Mr Kiir has told how he believes rebel forces are to blame for the growing hunger crisis in his country.
The UN classifies a famine as when 20 per cent of the population has fewer than 2,100 kilocalories of food a day and more than three in ten children suffer acute malnutrition due to not having enough food or water.
Mr Kiir said: “This is not our making. It is a man-made disaster and that is why we want the war to stop, to allow humanitarian access to the country.
“If Riek Machar was to understand that the civilian population is going to face one of the worst famines there has ever been, this is the time.”
Mr Kiir and Mr Machar signed a peace deal in Ethiopia on 9 May but accused each other of breaking the ceasefire shortly after it was meant to come into effect.
Speaking to BBC’s Hardtalk programme Mr Kiir said: “Mr Machar violated it, attacking our forces. He did not give orders to stop fighting after signing. My forces are observing the ceasefire strictly.”
He denied claims he had postponed the elections in order to ensure he stayed in power.
Instead, he claimed, US secretary of state John Kerry had asked him to postpone the vote to try to find an end to the conflict, which has left thousands of civilians dead.
The unrest has assumed ethnic overtones, with Mr Machar relying heavily on fighters from his Nuer ethnic group and Mr Kiir on those from his Dinka community.
The president’s warning came as UN humanitarian official Jan Egeland warned 7.3 million people could go hungry by August. Speaking at the start of an international aid conference in Norway to raise funds for South Sudan, Mr Egeland said: “I think $1.2 billion (£713 million) is needed of additional money to avoid us entering into a situation of famine, famine like we saw it a couple of decades ago.”
In addition to a looming famine, South Sudan is facing a cholera outbreak, according to international aid agency Oxfam.
The UN says hundreds of non-Nuers were killed in Bentiu last month when rebel forces captured the town.
Yesterday, peace talks between the rival groups were postponed until 29 May, amid continued disagreements over the presence of foreign troops and rebels, government officials have said.
Hussein Maar Nyuot, a member of the rebel delegation, said the latest postponement had been necessitated by the government delegation’s refusal to accept calls for the withdrawal of foreign troops which remain in the country against the provision of an earlier peace deal, signed in January. Mr Nyuot blamed the government’s “arrogance”.