SARAJEVO (AFP) - Bosnian police have impounded a pigeon after discovering prisoners used it to smuggle drugs into one of the country's highest security jails, an official said Thursday.

"The guards suspected the animal might be involved in drug smuggling once they noticed four prisoners visibly intoxicated shortly after the pigeon landed on a prison window," Zenica prison official Josip Pojavnik told AFP.

All four inmates had tested positive for heroin, said Pojavnik, adding disciplinary proceedings had been launched against the inmates.

The drugs, he added, had probably been stuffed into tiny bags attached to the legs of the carrier pigeon, which one of the prisoners had previously been allowed to keep as a pet in his cell.

"We suspect that the pigeon carried the drugs from Tuzla," a town around 70 kilometres (more than 40 miles) northeast of Zenica in central Bosnia, he added.

The pigeon had been taken into custody by police, who have launched an investigation aimed at identifying those who had loaded it up with the drugs.

"We do not know what to do with the pigeon, but for the time being it will remain behind bars," Pojavnik said.

The incident had prompted the prison administration to consider closing down a prisoner pigeon-breeding project established in a ward of the jail as part of a rehabilitation programme, he added.

Pojavnik insisted those birds had not been involved in the incident.

A similar case of carrier pigeons being used by prisoners was reportedly uncovered earlier this year at a jail in Brazil, where the birds were being used to deliver drugs and even mobile telephones.