Conditions in the Serbian province remain a mystery, but refugees have told relief agencies that the security forces have been destroying food stocks to make them leave.

Sergio Vieira De Mello, the UN Under Secretary-General for Humanitarian Affairs, said permission for the visit had been given through Yugoslavia's UN envoy.

Mr De Mello told a news conference that Vladislav Jovanovic had confirmed "his government would facilitate the conduct of the mission".

An advance team is preparing to leave for Belgrade. It will include representatives of the UN Children's Fund, the UN High Commissioner for Refugees and the UN's Office for the Co-ordination of Humanitarian Affairs.

UN Correspondent Mark Devenport: "Acceptance came in a letter"

It will make preparations for the arrival of a larger team, which intends to see conditions in Kosovo and other parts of Serbia.

The mission is expected to report to the secretary general by 13 May - the day Mr Annan has fixed for a meeting of all UN agencies involved in the Kosovo refugee crisis.

Figures unknown

UN relief agencies have not been in Yugoslavia since the Nato air strikes began on 24 March.

The Red Cross is also planning to work in Kosovo

The UN's refugee agency has expressed fears for the fate of displaced Kosovo Albanians still inside the province.

It is not known how many refugees are in Kosovo; neither journalists nor relief agencies have operated there since the Nato air campaign began in March.

Mr Annan said on Tuesday that he planned to send a UN mission to "assess the situation on the ground, determine the magnitude of the problem and what we need to do to prepare the ground for the refugees to go back to Kosovo".

He also said he had not asked for a pause or lull in the Nato bombing while the team was in the country but indicated some arrangements might be made.

Red Cross

The International Committee of the Red Cross has already been granted permission to resume operations in Kosovo.

Cornelio Sommaruga, President of the ICRC, visited Yugoslav President Slobodan Milosevic to get permission to work in Kosovo.