Marti Ahtisaari (r) insisted that the vehicles belonged to the UN

Martti Ahtisaari said he had tried to resolve the situation, which he said was sending the wrong message to the international community.

Eritrean officials say the vehicles were confiscated to try to pool resources and increase efficiency.

Aid agencies say it has hindered their humanitarian operations.

Two-thirds of Eritrea's 3.6m people rely on food aid.

Earlier this month the government backed down over a tax row which blocked imports of food aid at the port of Massawa.

Mr Ahtisaari, a former Finnish president and the UN's special envoy to the Horn of Africa, said the legality of the position was clear.

"If they are not needed any more for that particular purpose, then they are still UN property and the UN has the responsibility to keep them," he said.