The attackers who killed 10 African Union (AU) peacekeepers in the volatile western Sudan region of Darfur committed a war crime and should be investigated to bring them to book, Human Rights Watch (HRW) said.

"Deliberately attacking peacekeepers is a war crime," said Peter Takirambudde, HRW’s Africa director. "The Sudanese government and the rebel groups should cooperate fully with an independent investigation into the dreadful attack in Haskanita."

According to aid workers, unidentified men attacked an AU base in Haskanita, North Darfur, killing the men on 29 September. Some policemen were also attacked while other troops were kidnapped. Most were from Nigeria.

Describing the attack as heinous, the chairman of the AU commission, Alpha Oumar Konaré, said investigations were under way to identify the attackers.

Nigerian army spokesman Colonel Solomon Giwa-Amu said the incident could influence Nigerian policy in Darfur.

"It is unfortunate and tragic," he said. "We are investigating the incident. It is not totally unconnected with the failure of the mandate giving for the peacekeeping ... If the UN had come out stronger against the Sudanese government, we wouldn’t have lost as many soldiers at a time. We are not there for war."

UN Secretary-General Ban Ki-moon condemned the attack and called for the perpetrators to be held fully accountable for this outrageous act, describing it as "brutal and shocking".

This was the worst single attack that the 7,000-strong force had suffered in the region. Since 2004, the force has lost at least 25 men, with dozens of others injured, according to aid workers.

A 26,000-member hybrid UN-AU operation was authorised in July and is to be deployed early in 2008.

However, aid workers worry that the worsening situation in Darfur is making it increasingly difficult to adequately respond to the needs of the displaced. "We have a humanitarian problem that will not go away quickly," said John Distefano, ACT-Caritas Darfur emergency response operation director.

Among the worries are increasing clashes between and among armed groups representing differing tribal, ethnic and political groupings; banditry and ambushes and increased tensions in camps for the displaced.

According to the Church World Service, anxiety within the camps is heightened by long-standing fears of attacks and rape by militias; idleness and boredom; issues of civilian "protection" and when it might become safe for the displaced to return to their home villages.

More than 240,000 people have been newly displaced or re-displaced in Darfur during 2007, according to the UN Office for the Coordination of Humanitarian Affairs, with thousands of people fleeing their homes each week.