Attackers shot up an ambulance and wounded three Blue Helmets during an ambush in Sudan's Darfur region, the UN's peacekeeping chief said on Thursday.

Herve Ladsous, speaking after a visit to East Darfur, said the attack by "an armed group" against the African Union-UN Mission in Darfur (UNAMID) happened in the community of Labado.

"And what is even worse is that an ambulance which was with the convoy, an ambulance showing very clearly the red crescent, was shot at. And I could myself touch the holes of the bullets in the ambulance," Ladsous, the UN's Undersecretary General for Peacekeeping, told reporters in the capital Khartoum.

"This is absolutely unacceptable."

In meetings with Sudanese government officials Ladsous said he called for "full cooperation" to determine who carried out the attack.

"I told my interlocutors that I very much hope that this investigation will move forward, and would lead to firm conclusions," he said.

"It is simply unacceptable that we get attacked time and again and we are never able to say who is responsible."

In April, a Nigerian UNAMID peacekeeper was killed and two others were wounded in an assault against their base near Muhagiriya town which is in the same area more than 60 kilometres (37 miles) east of the South Darfur state capital Nyala.

At the time, local sources said the fatal attack appeared to have been planned and carried out by government-linked forces, but authorities denied such a connection.

More than 40 peacekeepers have been killed in hostile action during UNAMID's five-year history and the UN has repeatedly called for perpetrators to be brought to justice.

However, UN sources have said they were unaware of anybody previously being held accountable in Sudan for killing a peacekeeper.

August arrival of first UN troops for Sudans' border
Khartoum (AFP) July 04, 2013 - An advance party of UN troops who will support the monitoring of a buffer zone along the disputed Sudan-South Sudan border should arrive by August, the top UN peacekeeper said on Thursday.

They will be among more than 1,000 Blue Helmets who will eventually protect the border monitors, Herve Ladsous, the UN's Undersecretary General for Peacekeeping, told reporters in the Sudanese capital.

"We are in the process of deploying an additional battalion," he said.

While the first troops are expected next month, the rest "will be deployed as quickly as possible," to activate the border monitoring, Ladsous added.

"Because that is a crucial way to contribute to confidence between the two parties on that very long, 2,200 kilometres (1,364 miles), and very complicated border."

The troops with special border duties will be part of the UN Interim Security Force for Abyei (UNISFA), one of the contested border regions.

After months of intermittent clashes, Sudan and South Sudan agreed in early March to detailed timetables for normalising relations by setting up the border buffer zone and implementing eight other key pacts, including an oil deal.

Last month Khartoum said it was suspending all the agreements over alleged South Sudanese support for rebels in the north.

But vice-presidents of the two countries met in Khartoum this week and committed to ease tensions.