Three months ago, Unita issued a declaration of complete demilitarisation. As a result, the movement was legalised as a political party.

However, the UN mediator in Angola, Alioune Blondin Beye, said his observer mission had for the first time gathered proof that Unita has retained a military force in several parts of the country.

Unita is still occupying four key towns

Mr Blondin Beye said he now expected to travel to New York at the end of this week for consultations with the UN Secretary General Kofi Annan.

Meanwhile, the UN said Unita had proposed a new date for the handover of four remaining Unita strongholds in central Angola.

Unita is said to have offered to transfer the areas of Andulo, Bailundo, Nhareya and Mongo to government control by June 25.

Political inauguration

The news that Unita had failed to disband all of its army comes after an 18-month demobilisation process which cost the UN several hundred million dollars.

It emerged just as the movement was re-inaugurating its political headquarters in the capital, Luanda.

BBC correspondent Anna Richardson said the event was intended to mark the movement's transition from a military organisation to a political opposition party.

The ceremony should have been highly symbolic.

Unita's office building was completely destroyed in 1992 when Angola's civil war resumed. Several of the movement's top officials were killed within its walls.

Under the terms of Angola's 1994 peace accord, the re-inauguration should have coincided with the completion of the country's peace process.

Our correspondent said that in reality it has coincided with a deepening crisis in the peace negotiations.