More than four years after an African Union-UN peacekeeping force costing billions of dollars arrived in Sudan's restive western region of Darfur, peace remains elusive and some question the mission's value.

Critics say the United Nations-African Union Mission in Darfur (UNAMID), the world's largest peacekeeping operation, is too close to the Sudanese government and not aggressive enough in fulfilling its core mandate of protecting civilians.

"It may be better than nothing," one analyst said, asking for anonymity. "But they are really focused on protecting themselves."

The concern comes as Darfur suffers a surge in violence.

More than 700 people have already been killed this year in clashes between rebels and government troops as well as in tribal unrest and criminal incidents, more than for the whole of 2011, UNAMID data show.

Rebels drawn from black African tribes rose up against the Arab-dominated Khartoum government in 2003. The conflict peaked years ago, killing at least 300,000 people, according to UN estimates. The government said 10,000 died.

Overall, there has been a "drastic decrease" in the number of people killed in clashes, and things would have been worse without UNAMID's roughly 16,700 troops, the force commander says.

"The mere presence of us on the ground flying the flag is a substantial deterrent," Lieutenant General Patrick Nyamvumba of Rwanda said.

Since he took over three years ago, UNAMID patrols have roughly doubled to around 150 a day, and more people displaced by the conflict have returned home.

The UN recorded 178,000 returnees between January 2011 and March this year.

"This could not be possible if there were not increased security," Nyamvumba said.

But an estimated 1.7 million remain in camps -- which more closely resemble poor villages -- where residents have reported shootings, arson and other violence.

In a July report focused on Khartoum's use since late 2010 of non-Arab militia to displace ethnic Zaghawa rebels and civilians from east Darfur, Swiss-based independent researchers alleged that in several cases "abuses against civilians, looting, and burning of property occurred in the immediate vicinity of UNAMID positions".

Nyamvumba says his troops are clearly mandated to give "physical protection" to civilians in imminent danger, which they have done.

The fact that 38 UNAMID peacekeepers have been killed in hostile action shows they are doing their job, he said, adding: "I think the mission has accomplished quite a lot."

'They were not able to protect themselves'

But Darfur's top official, Eltigani Seisi, said the protection mandate seems to lack clarity.

"We saw incidences where UNAMID forces have been attacked and they were not able to... protect themselves," he said.

UN figures show that 13 UNAMID vehicles were carjacked in the first half of this year.

"If they are not able to protect themselves, they cannot protect the civilians," said Seisi, who heads the Darfur Regional Authority, set up to implement a peace deal signed last year between Khartoum and rebel splinter factions.

Key rebel groups have rejected the agreement.

"They have the guns and they don't use them," a humanitarian source said of UNAMID.

Among the peacekeepers' arsenal are five Mi-35 helicopter gunships used mostly for reconnaissance and escorting patrols, said Christopher Cycmanick, a mission spokesman.

"They haven't been engaged in any fighting," he said. "They're there more as a deterrent."

Another 22 helicopters are for medical missions and logistics.

"In fact, they are doing nothing," the humanitarian source said, asking for anonymity. "They are just providing transport."

He said UNAMID has become "part of the problem" in Darfur and charged it is "very close" to the government.

The Swiss-based researchers, Small Arms Survey, referred to "widely held suspicions by Darfuris within and outside Darfur that UNAMID is biased towards the government".

Nyamvumba countered that UNAMID is "committed to being impartial", trying to interact with both Khartoum and the rebels whom it seeks to bring into the peace process.

"When you deploy a mission of this kind, it comes with a lot of expectations," Nyamvumba said. "What I can assure you is we do what it takes, and what we can, to assure that the mission accomplishes its mandate."

Seisi said he did not believe UNAMID receives "instructions" from the government.

The UN Security Council on July 31 expressed "deep concern at increased restrictions and bureaucratic impediments placed by the government of Sudan upon UNAMID movement and operations, particularly to areas of recent conflict".

Seisi agreed "there are some issues with access" that the government of President Omar al-Bashir, who is wanted by the International Criminal Court for war crimes allegedly committed in Darfur, is trying to address.

UNAMID's budget is about $1.4 billion for 2012-13 while Darfur, where villages were razed during the conflict, still awaits major funds for rehabilitation.

The criticisms of UNAMID are unfair, an African diplomat said, noting it must satisfy two bosses -- the AU and UN -- and navigate possibly contradictory tasks including mediation and protection.

"It's not so simple on the ground," where the peacekeepers operate in a vast territory accounting for about one-third of Sudan. "It's a mission impossible," the diplomat said.