The investigation comes as Khartoum declared a unilateral ceasefire in its war with southern rebels - to last for two months.

The UN teams are to conduct health tests on local people to see if they have been exposed to chemical agents.

The military-backed government in Khartoum has denied the allegations made by the rebel Sudan People's Liberation Army, which said 22 bombs were dropped from a high altitude on the towns of Lainya and Kaya in Western Equatoria province nearly two weeks ago.

A government minister on Wednesday said "hidden agendas" were operating behind the UN investigations.

President Omar al-Bashir: A change in position

Hassan Dahawi, Sudan's state minister for social planning, told the independent Al-Rai Alaam newspaper that certain unnamed UN officials were acting "under the influence of some hidden agendas that have nothing to do with humanitarian work".

Earlier this week Sudan denied the SPLA allegations as "lies" and "an aggressive act against Sudan".

A Norwegian relief agency working in rebel-held areas said one of its doctors had reported children and adults vomiting blood, and aid workers from the World Food Programme have complained of burning sensations in their noses and eyes.

The UN World Food Programme said some of its people were hurt and were "recovering" - but added that it could not comment until its investigation was complete.

At least 1.5 million people have died in Sudan's 16-year civil war between the Khartoum-based Islamic government and rebels in the south.

Sudan Government ceasefire

A Sudanese Foreign Ministry statement said the ceasefire would begin at midnight on Thursday and was designed to help facilitate relief operations.

A spokesman for the rebel Sudan People's Liberation Army, Samson Kwaje, dismissed the move as a "trick" intended to deceive the rebels.

Last month, the rebels declared a three-month ceasefire in three provinces, covering 70% of southern Sudan.

But the government rejected it as a propaganda move.