Health authorities in Amsterdam are warning of dangerous drugs being sold to tourists after three British men died in the last month after snorting heroin they may have believed was cocaine.

A large sign outside the Dutch capital's Central Station warned Thursday, "Extremely dangerous cocaine is sold to tourists" in the area.

The warning campaign was started after two British men, aged 20 and 21, were found dead in a hotel room in the city on Tuesday. Police say they likely died after snorting "white heroin" they likely bought believing it was cocaine.

Amsterdam police spokesman Rob van der Veen says the same drug is suspected in another British tourist's death last month.

Van der Veen says it is not yet clear why heroin is apparently being sold as cocaine.