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A deadly firefight in Sudan's Darfur this week proves that Blue Helmets can "do the job" in the region where security has deteriorated, the UN's top peacekeeper told AFP in an interview.

Herve Ladsous was responding to critics who say the African Union-UN Mission in Darfur (UNAMID) -- one of the largest peacekeeping operations in the world -- is not aggressive enough in fulfilling its mandate to protect civilians.

At the same time, he admitted there have been a few cases where the peacekeepers' actions did not meet expectations.

On Wednesday, though, a Nigerian patrol "performed very well" during an ambush by unknown gunmen in the community of Labado, east of the South Darfur capital Nyala, he said.

"Let's face it, they can be aggressive and the incident in Labado yesterday showed it", the Undersecretary General for Peacekeeping told AFP late Thursday in Khartoum.

"They did kill at least one, maybe more, but we did find the one body and that shows that, yes, we can do the job."

Three of the peacekeepers were wounded and an ambulance with their patrol was shot up, an "absolutely unacceptable" act, said Ladsous.

He visited Darfur to assess UNAMID operations, and held talks with Sudanese officials including President Omar al-Bashir.

A diverse group of critics from rebels to Darfur's top official, Eltigani Seisi, have expressed concerns about UNAMID's ability to safeguard the population, including 1.4 million displaced by Darfur's decade-long conflict.

Six UNAMID peacekeepers, five of them from Nigeria, have been killed in attacks in Darfur since last October. More than 40 peacekeepers have died in hostile action since the mission's establishment in 2007.

Rebels have been fighting for 10 years in Sudan's far-western Darfur but the UN says worsening inter-tribal clashes have been responsible for most of the violence this year.

UNAMID was set up under Chapter VII of the United Nations Charter, which allows for the use of armed force.

The mission "has the inherent robustness to deal with the situation," Ladsous said at a separate news conference.

"It is true, though, that we have experienced some incidents in which the reaction of the troops was not exactly what we would have expected," he said.

The UN is working with nations providing troops to ensure that training and equipment meet UN standards, Ladsous added.

UNAMID has about 20,000 military and police officers.

But it lacks helicopter gunships such as those which a UN mission in the Democratic Republic of Congo used last year against advancing rebels.

UNAMID sent back its five Mi-35 tactical helicopters to Ethiopia last year.

"We didn't really have a direct need for them," Ladsous told AFP in the interview, explaining they were not suitable against small groups of individuals in light vehicles, as is typical of the Darfur conflict.

"That is why we put a lot of emphasis on ground patrols," he said, adding that UNAMID needs more transport helicopters, some of which are expected from Indonesia soon.

A UN source last year told AFP the Sudanese government sometimes did not give clearance for the Ethiopian gunships to fly.

UN Secretary General Ban Ki-moon has repeatedly noted Sudanese government restrictions on UNAMID land and air movements, though these affect a small percentage of patrols.

The mission is entitled to "full and unrestricted freedom of movement" under an agreement with Khartoum.

In April, Ban also reported a backlog of more than 1,400 visas for UNAMID personnel, mostly police, and delays in the clearance of UNAMID cargo.

Ladsous says the government has more recently given "better cooperation", particularly for the visas.

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

"I cannot accept impunity, either of crimes against civilians or of crimes against peacekeepers," Ladsous told AFP.

"It is true that we have had little by way of information after past investigations" by Sudanese authorities into the attacks.

Ladsous hopes the Labado case will be different.

"Where we have a body, which is being handed over to the appropriate Sudanese authorities, I do expect information about who that was, which group did he belong (to), and followup," he said.