More than 250,000 children in war-torn South Sudan are "at risk of imminent death" because of severe malnutrition, a United Nations official has said.

Henrietta H Fore, executive director of the UN children's agency (UNICEF), issued the stark warning on Friday after a two-day visit to some of the areas most affected by the country's civil war, now in its fifth year.

"It's serious here in South Sudan," she told Al Jazeera from South Sudan's capital, Juba.

"We are very worried that a quarter of a million children are going to be facing death this year before July."

The war, which broke out in 2013 after President Salva Kiir accused his former deputy Riek Machar of plotting a coup, has devastated agricultural production in the East African country, she said

"The violence has meant that many of the farmers have run away from their fields," said Fore. "They are afraid to farm, and as a result there is just no food in the markets."

Calling for emergency action to boost food security, Fore said South Sudan was now heading into the dry season, which "means there's just less food, less water to be found".

"The acute and severe malnutrition is growing stronger," she said.

The conflict, which has resulted in the death of tens of thousands and the displacement of a quarter of the country's 12 million population, has also affected more than half of its child population, UNICEF said.

Some 2.4 million children have been forced to flee their homes since the war broke out. More than 2,300 children have been killed, and 19,000 have been recruited into armed groups.

The agency also said it has documented more than 1,200 cases of sexual violence against children.

More than 70 percent of children are not getting an education because at least one in three schools have been damaged or closed, it added.

Despite the catastrophic humanitarian crisis, aid agencies say the delivery of relief services has been complicated by attacks on humanitarian workers - 28 officials were killed last year alone.

In December, South Sudan's warring sides signed a ceasefire deal in Ethiopia's capital Addis Ababa, and agreed to allow humanitarian aid to get to civilians caught up in the fighting.

But the truce has been repeatedly violated, with both sides blaming each other for the breaches.