Sudan 'blocks aid' to civilians in rebel-held Blue Nile
Sudan is blocking aid to a rebel-controlled area, putting the lives of the population at risk, the head of the SPLM-North rebels has told the BBC.
More than 200,000 people are in dire need and elderly and children are already starting to die in the Blue Nile state, Malik Agar said.
The rebels signed a deal this year with the UN, African Union and Arab League to allow aid into conflict areas.
Many thousands have been been displaced along the border with South Sudan.
More than 70,000 civilians from Blue Nile are being cared for by the UN refugee agency in South Sudan, which gained independence last July.
Tensions between the two neighbours, who fought a long civil war which ended in 2005, have ratcheted up over the last month, with fears of an all-out war.
Wednesday is the deadline set by the UN Security Council for both sides to pull their forces from the border and activate a joint monitoring mission.
Mr Agar called for urgent international assistance for about 200,000 people in Blue Nile state, just north of the border with South Sudan.
"The situation is catastrophic and the vulnerable, the elderly, pregnant women and children, are dying on a daily basis," Mr Agar, the chairman of the SPLM-North told the BBC.
He warned the fast-approaching rainy season will worsen the situation.
Humanitarian groups, including Samaritan's Purse, have visited the area and say homes have been bombed and families, prevented by the fighting from farming, have been reduced to eating leaves and roots.
Many people live in caves in the hills to avoid aerial bombing, which happens day and night.
The agreement - a document seen by the BBC - to deliver food aid was signed three months ago, but so far no deliveries have got through because Khartoum has denied access and was blocking the plan, Mr Agar said.
BBC Africa analyst Martin Plaut says there has been pressure for food to be sent into conflict areas, even without approval from Khartoum, much as it was during South Sudan's long years of fighting for independence.
The SPLM-North say they are waging a war in the states of South Kordofan and Blue Nile to change the government in Khartoum, not to establish another independent state.
They see themselves as continuing in the footsteps of the movement from which they sprang, the SPLM of the late John Garang, which now runs the newly independent state of South Sudan.
When independence came in July last year, many SPLM forces in Blue Nile and South Kordofan were left stranded in Sudan.
These areas were supposed to have been allowed a vote to choose autonomy, but this was blocked by Khartoum.
The SPLM-North routinely denies receiving support from South Sudan, and the government denies any connection with the rebels.
Juba signed an agreement with Khartoum not to support rebellions in each other's states, but there are strong suggestions that both sides flout this pact.
There are also concerns about the humanitarian situation in refugee camps in South Sudan.
UK charity Oxfam has warned that the Jamam camp in Upper Nile state is experiencing water shortages and congestion, which is making it difficult for aid distribution.
In neighbouring Unity state, aid workers in Yida refugee camp say there has been a big rise recently in the number of people fleeing fighting in South Kordofan.
"In just a couple of months, the camp population has nearly doubled and is about to hit the 30,000 mark," the International Rescue Committee told the BBC in a statement.
"The sudden surge in new arrivals is putting a huge strain on resources. Many refugees are struggling to find enough wood and grass to build shelters, while more medical supplies are needed to treat the newly-arrived, many of whom suffer from malnutrition and exhaustion."
Meanwhile, the UN High Commissioner for Human Rights Navi Pillay, begins talks on Wednesday with South Sudan's President Salva Kiir over the humanitarian situation in his country.
The UN has expressed concerns over human rights violations as tensions rise between Sudan and South Sudan