The United Nations said on Thursday it was struggling to reach 18-million people in need in Sudan to fend off a humanitarian disaster, hampered by insecurity, interference from the warring parties and a lack of international support.

"We need to reach 18-million people and we will not give up on that target, but we need more international support, better access to the people who need us and safety for our operations," said Clementine Nkweta-Salami, Deputy Special Representative of the UN Secretary-General and Resident and Humanitarian Coordinator in Sudan.

"We're not at this point able to have an operation that is commensurate to the scale (of needs)," she said.

Conflict between the Sudanese army and the paramilitary Rapid Support Forces (RSF) erupted in mid-April, sparking a grave humanitarian crisis and pushing the country to the brink of civil war.

More than 4.2-million people have fled their homes because of the war, and nearly 1.2-million have crossed to neighbouring countries, putting huge pressure on Sudan's meagre resources.

Nkweta-Salami said that 19 aid workers had been killed and 29 injured since the fighting broke out, which she said was unacceptable and unlawful.

Red tape was hampering the distribution of humanitarian aid, and a $2.6-billion humanitarian appeal to the international community to help the people of Sudan was just one third funded, she said.

"We also need to see an end to interference from the conflict parties in our operations, including forced checks of humanitarian trucks and mandatory military presence during the loading process in Port Sudan and Jazeera," she said.

"I hope that we will see swift action to reduce the bureaucratic obstacles, including delays in visa approvals for staff as promised by Sudanese authorities in a donor meeting yesterday."

The UN fears that the crisis could worsen if the conflict spreads to other areas such as central Jazeera state, which she called "Sudan's breadbasket", where fighting could hinder food supplies. "This will have great consequences for food security."