The top U.N. humanitarian official in Sudan on Tuesday called for immediate access to those wounded in a government bombing attack on a school and busy marketplace in North Darfur, which killed 12 people.

Around 30 wounded, mostly women and children, have been waiting for help since the bomb attack on Sunday. No help has arrived because of logistical difficulties and denial of access by authorities, according to witnesses and aid sources.

The attack killed 12, including six children, a teacher at the school told Reuters by telephone. Earlier reports had figures of 13 killed.

"I am deeply perturbed by the reported bombings of a school, water installations and a market where civilians, especially women and children are present," Sudan's U.N. Humanitarian Coordinator Ameerah Haq said in a statement.

"It is essential that we have immediate humanitarian access to the affected populations in order to provide humanitarian assistance and medical evacuation."

Haq is in Norway where donor nations are meeting Sudanese officials to pledge money to rebuild Sudan after decades of multiple civil wars.

Mokhtar Mohamed, a teacher at the school in Shegeg Karo, said they had been told many times that U.N. planes were coming to evacuate the wounded but no help had arrived late on Tuesday.

"They said the government had stopped any planes coming to Shegeg Karo this morning," he said.

One aid source in Darfur also told Reuters that the government had not allowed planes to fly in on Tuesday. Sudan's army did not answer calls seeking comment.

If the death toll from the attack is confirmed, it would be the single most deadly bombing raid in many years in Darfur.

The U.N. children's agency (UNICEF) urged the parties to the conflict to take all necessary measures to protect children.

"Children are to be afforded special protection during military operations and ...every effort must be made to ensure that schools, health centres...are considered as zones of peace, UNICEF said in a statement on the bombing raids.

Medical care even in the state capitals of Darfur is basic and it is practically non-existent in remote rural areas like Shegeg Karo.

International experts estimate some 200,000 have died and 2.5 million driven from their homes in five years of revolt in Darfur. Washington calls the violence genocide, a term Khartoum rejects and European governments are reluctant to use.

Sudan blames the Western media for exaggerating the conflict and puts the death toll at about 10,000.