By Jeremy McDermott BBC correspondent in Colombia

The Colombian authorities have launched an investigation into expenditure at the anti-narcotics police after the US suspended aid to the unit, alleging that some $2m had been diverted from a bank account provided to cover administrative expenses.

A US Embassy source said that further funds to the account would not be released until the matter had been resolved.

The head of the anti-drug police, General Gustavo Socha, said that several officers had already been retired over the matter and that the anti-narcotics operation would continue unhampered.

The incident is embarrassing for both the Colombian and US governments as President Bush increases aid to this beleaguered South American nation - now the third largest recipient of American military aid in the world after Israel and Egypt.

Mr Bush wants to broaden his war on terrorism to include Colombia, as three of the country's warring factions are on Washington's terrorism list.

However, accusations of police corruption will provide more fuel to opponents of further involvement in Colombia's bloody civil conflict.

Several US congressmen have said there is little to show for the billion dollars of aid former President Bill Clinton granted to Colombia.

And, they say, further commitment could simply drag America into another Vietnam.