UN Under-Secretary-General Benon Sevan said he was satisfied with President Jose Eduardo dos Santos commitment to help find the planes.

Benon Sevan: Satisfied with the Angolan commitment

After a meeting with the president Mr Sevan said he was happy with the government's "full support and commitment to assist us in sending in an investigation team".

The envoy was vague when asked if he had had contact with Unita.

"I am here to hold discussions with the government of Angola. We have reached a rather sensitive stage in the discussions," he said.

Mr Sevan said final arrangements would be worked out when he met the Defence Minister, Pedro Sebastaio, later on Wednesday.

Both planes came down near the central city of Huambo, an area held by Unita rebels. There has been fierce fighting in the region between government and rebel forces over recent weeks.

The government and Unita have blamed each other for shooting down the planes. Unita has denied government claims that it is holding any captured survivors.

No ceasefire

The UN sent Mr Sevan to Angola to facilitate rescue operations for the planes, but he has so far failed to secure a ceasefire to allow a full search for survivors.

The government told the UN on Tuesday it could not agree a ceasefire while Unita rebels continued military operations in the centre of the country. Unita declined to comment on the ceasefire request.

Since the downing of the planes, the UN has halted all flights in and out of Angola and withdrawn its staff from combat areas.

Mr Sevan said that President dos Santos also promised to provide security and safety for humanitarian personnel working with the UN and other aid agencies in Angola.

UN 'will pull out'

Fighting between Unita and the Angolan Government erupted in early December, marking the collapse of a precarious four-year-old peace accord intended to end more than 20 years of civil war.

Earlier this week a government spokesman confirmed suspicions that the UN mission would pull out all of its observers by 26 February, when its mandate is officially due to terminate.

A BBC correspondent in Luanda, Lara Pawson, says many Angolans fear that if the UN mission does pull out, the civil war will rapidly escalate into a bloodbath.

Both UN planes crashed shortly after take-off from Huambo. The first, with 14 people on board, went down on 26 December. The second, carrying eight people, crashed on Saturday.