Malik Agar, chairman of the Sudan People's Liberation Movement/Army-North (SPLM/A-N), said aerial bombardment by the government had killed 74 civilians and wounded more than 100 in Blue Nile state in southeast Sudan since clashes began on 2 September. He further called for the government to stop bombing its own citizens.
The Sudanese government dismissed Agar's account. It has blamed rebels for causing hardship for citizens in border areas like Blue Nile and insists it is protecting its people.
Fighting also erupted in June in the Nuba Mountains in South Kordofan, another state that, like Blue Nile, borders newly independent South Sudan. Aid agencies have been unable to operate in either region since the conflicts began.
"There are basic things that we are asking for: one is for pressure to be exerted on Khartoum to stop the bombing of the civilians," Agar told reporters at his temporary command headquarters in a rebel-held area of Blue Nile.
Agar, the elected SPLM governor of the state, was sacked after the fighting erupted and a military ruler was appointed. The SPLM in the north was also shut down. The SPLM is South Sudan's ruling party and has an offshoot in Sudan.
"We are asking and demanding that they open corridors and safe areas for the humanitarian operations," he said, speaking on Thursday during a trip for reporters arranged by rebels based in South Sudan.
Agar, speaking near Kurmuk which is about 160 km (100 miles) from Blue Nile's capital Damazin, said as many as 500,000 people may have fled villages to the bush and would soon run out of food. That number could not be verified.
The United Nations Refugee Committee says over 27,500 people have fled Blue Nile to Ethiopia in the last month.
Information Ministry spokesman Rabie Abdelaty told Reuters by telephone that 95 per cent of civilians living in Blue Nile were safe and the government was providing assistance.
Reuters spoke to several families during the trip, who said they had abandoned their villages to avoid bombardment by Sudanese government planes. They were living rough under trees and gathering food in the wild to survive.
Abdelaty dismissed the accounts, saying they were made by Agar's supporters.
Agar said President Omar Hassan Al Bashir, who is accused of war crimes in Darfur in west Sudan, was using food as a weapon and said the "catastrophic" humanitarian situation needed swift intervention by the United Nations.
"There are no safe havens for them because the main problem for them is Soviet-made high-altitude bombers. You just witnessed two of them hovering over us. The bombs were dropped on a village, not near any military installation," he said.
This correspondent saw planes circle the area and drop bombs about a kilometre (mile) away. It was not immediately possible to determine the type of planes involved.
Abdelaty said the government was not involved in bombarding civilian areas and said any demand for international humanitarian assistance could only come from Sudan's government. Khartoum has denied it has used food as a weapon.
Agar urged the international community to push Bashir to negotiations saying "wars end on tables." He said fresh talks must be mediated by a third party, a move Bashir has ruled out.
Agar's forces fought alongside southern rebels during a brutal 22-year civil war that ended in a 2005 peace deal that paved the way to southern independence on 9 July this year.
"I was an elected governor who was removed unconstitutionally, and our party was a party that ran elections with a considerable number of MPs, 128 of them are all in prisons," Agar said, wearing military fatigues.
Both sides gave conflicting accounts of how much land they controlled. Agar claimed his forces controlled around 80 per cent of land in South Kordofan and Blue Nile. Abdelaty said government forces control 95 per cent of the land in Blue Nile.
The SPLM-N is forging closer alliances with insurgents in the troubled Darfur region and in the east, with the intention of launching a coordinated attack on Khartoum, he said.
"We had a discussion with them of how to coordinate our forces and how to coordinate our military operations on the ground," he said. "I am interested in taking Khartoum."