BBC Correspondent Jim Fish: The UN could pull out of Angola altogether

The two planes came down within eight days of each other near the central city of Huambo, where government and rebels forces have been engaged in heavy fighting.

The government says Unita rebels shot down the aircraft and is holding survivors captive, but this has been denied by Unita.

The leader of the UN team, Under-Secretary-General for Security Benon Sevan, said it was unacceptable that there was still no independent confirmation of the whereabouts or condition of the 22 people on board the two planes.

Benon Sevan: Unacceptable situation

"We are talking about the lives of people who have come to this country to help bring peace. They deserve more," he said.

Mr Sevan is attempting to get the government and Unita to agree a full ceasefire so that some kind of search may at last begin for the missing aircraft.

He said the UN security council was deeply concerned at the manner in which the government and Unita rebel leaders handled both crashes.

The government has agreed to co-operate with the UN observer mission, but Mr Sevan said writing letters of co-operation was not enough.

Unita has said the UN has not asked it officially for any help. Mr Sevan will travel to Huambo on Wednesday, but it is not clear whether he will meet Unita leaders. He has said he has messages for both the government and Unita.

Within a few hours of arriving in the capital Luanda on Monday morning, Mr Sevan met with the most senior members of the UN observer mission. His team is due to stay in Angola two or three days.

The two planes crashed less than 50km apart. The latest plane to go down was carrying four UN personnel and four crew. It crashed shortly after take-off from Huambo. Ten UN officials and four crew were on board the first aircraft, which went down a week ago.

The UN has suspended flights around Huambo and decided to withdraw all its 1,000 observers from areas of combat. UN Special Envoy Issa Diallo said it appeared the UN was being targeted.

Huambo - Angola's second largest city - is held by the government and is used by the army to fly in reinforcements for fighting in surrounding areas.

Angolan National radio says fighting has now spread to the government controlled city of Malanje. At least 12 people are reported to have been killed during rebel shelling of the town.