With war raging in separate conflicts on both sides the Sudan-South Sudan border, civilians have nowhere to run and hunger is approaching famine levels, the United Nations said.
South Sudan became independent from Sudan in 2011, following a peace deal that ended decades of civil war between north and south. But the new nation became embroiled in its own internal conflict in December 2013. Meanwhile, fighting has intensified between the government and rebels in Sudan's southern states of South Kordofan and Blue Nile.
Hungry civilians caught in the crossfire on both sides are struggling to find places of refuge where humanitarians can deliver emergency aid.
"We are seeing significant inter-connections between the humanitarian situations between the two countries," Valerie Amos said after briefing the U.N. Security Council on Wednesday.
"The conflict in South Sudan has effectively blocked off traditional areas of refuge across the border. It has also disrupted the cross-border movement of goods and services coming into South Kordofan and Blue Nile."
The wars in Sudan's South Kordofan and Blue Nile states date back to the 1983-2005 civil war when rebels of the Sudan People's Liberation Army - North (SPLM - N) fought on the side of the southern rebels of the SPLM against Khartoum.
When South Sudan was given independence and the SPLM formed a government in Juba, the SPLM-N remained part of Sudan. They took up arms against Khartoum again in 2011. Human rights groups have accused the Sudanese military of carrying out indiscriminate air bombardments against civilians in rebel-held areas such as the Nuba Mountains.
The U.N. estimates that 170,000 people have been displaced in within SPLM-N areas in the first half of 2014.
Hunger is at the level of 'emergency' - phase four out of five on a sliding scale where five equals famine - in rebel-controlled areas of South Kordofan, Amos said. This means that there is "very high malnutrition or excess mortality", according to the Famine Early warning Systems Network.
Intensive bombing campaigns have prevented people from farming. Instead, they hide in mountainous caves for safety.
In June, a clinic run by the medical charity Medecins Sans Frontieres in South Kordofan was bombed, in violation of international humanitarian law.
Khartoum has denied humanitarian access to hundreds of thousands of people in rebel held areas since the war resumed three years ago.
In Sudan, humanitarian agencies require permits from multiple government agencies to travel. These can take weeks to obtain.
Sometimes they are not granted at all, particularly to areas of conflict where the government says it cannot guarantee the safety of aid workers.
"The government has not allowed any humanitarian assistance to be taken over the front lines from government held areas into non-government held (SPLM-N controlled) areas in South Kordofan," Damian Rance, Khartoum spokesman for the U.N.'s Office for the Coordination of Humanitarian Affairs (OCHA) told Thomson Reuters Foundation in an email.
"Government permission has also not been granted for any cross-border aid to travel from South Sudan into SPLM-N controlled areas either."
In the past, there have been reports that non-governmental organisations have been illicitly transporting aid into these areas from South Sudan. This traffic has been disrupted since war broke out in South Sudan.
In addition, no polio vaccinations have been carried out in rebel territory as part of a Sudan-wide campaign for children under five.
"Although Sudan was recently declared polio free, cases of polio have begun to re-appear throughout eastern Africa and the risk of the virus resurfacing in Sudan is high," said Rance.
The World Food Programme (WFP) provides food to government-controlled areas, such as South Kordofan's state capital, Kadugli.
"These are the vulnerable people. Some of them have fled SPLM-N-held areas. We are trying to reach at least 150,000 of them," WFP's spokeswoman in Khartoum, Amor Almagro told Thomson Reuters Foundation by phone.
These hungry communities are now playing host to some 83,000 South Sudanese who have fled northwards to escape fighting in their home country, Amos said.
White Nile is another Sudanese border state, sited between South Kordofan and Blue Nile.
The situation in South Sudan has become so bad in recent months that more than 2,000 Sudanese refugees have returned home to Blue Nile State in search of food.
In South Sudan's camps, malnutrition rates among some 200,000 Sudanese refugees have risen because fighting is blocking food deliveries from the capital, Juba. Hungry children have resorted to eating flying ants.
Ironically, Sudan is now helping to feed its former enemy.
On July 8, the two governments signed a deal allowing WFP to transport food from Sudan to South Sudan's Upper Nile, Unity and Jonglei states.
Agencies have warned that famine could break out in these areas, which have oil reserves and have been worst affected by the war. Four million South Sudanese, more than a third of its population, are facing emergency levels of food security.
Editing Ros Russell; email@example.com