European Union environment officials have determined that two kinds of genetically modified corn could harm butterflies, modify food chains and disturb life in rivers and streams, and they have proposed a ban on the sale of the seeds, which are made by Pioneer Hi-Bred, Dow Agrosciences and Syngenta. The preliminary decisions, seen by the International Herald Tribune, are circulating within the European Commission, the EU executive, which has the final say. Some officials there are skeptical about a ban that would upset the powerful biotechnology industry and could exacerbate tensions with important EU trading partners like the United States.