By Justin Pearce BBC, Angola

An incident involving an anti-tank mine has raised new concerns about the security of the humanitarian operation in Angola.

A truck carrying emergency food aid was damaged when it detonated the mine in the centre of the country.

The accident came as a surprise.

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For several weeks now, aid vehicles, army trucks and some commercial lorries have been safely using the road from the central Angolan town of Kuito to Ndele, about 50 kilometres (31 miles) to the south.

Ndele is one of the areas where soldiers of the former rebel movement, Unita, have assembled for disarmament.

But a United Nations official confirmed that a truck carry aid supplied by the World Food Programme detonated an anti-tank mine on that road earlier this week.

Road closed

The food was intended for the families of Unita soldiers at Ndele.

No-one was hurt in the incident, but the vehicle was damaged.

"We have closed the road and asked a demining company to look at it so we can use the road as quickly as possible again," UN Humanitarian Co-ordinator Erick de Mul told the BBC.

The Ndele route has seen regular traffic by military and humanitarian vehicles in recent weeks. But Mr de Mul ruled out the possibility that the mine had been laid recently.

He said that the device had probably been buried deeper than normal, which is why previous traffic on the road had not detonated it.

"Before it sets it off the ground has to be compacted and the way it becomes compacted is by trucks or cars going over it," he said. I think this is a typical example of what happened there."

Funding appeal

Since the peace deal was signed in April, aid operations have been moving into areas that were previously off limits.

But mines and broken bridges are still preventing emergency supplies from reaching many of the people in need.

Although the process of demobilising Unita is officially in its final stages, there is no reliable record of what weapons the rebel soldiers still hold.

Emergency aid is now reaching a greater number of people displaced by the war in Angola, and there have been several appeals for donor funding.

But aid officials say that logistical and security problems are still a major obstacle in the way of reaching people in need.