Israel, which along with the United States conducted the most sophisticated cyberattack in history, the Stuxnet attack on Iran’s nuclear enrichment program, also declined to sign.

The United States did not say why it did not sign the accord, and one diplomat, who would not allow the use of his name because he is not authorized to speak publicly on policy issues, said that it was possible that Washington may sign the principles at a later date.

But if past debates over setting norms of behavior for cyberspace are a guide, American officials are leery of any kind of agreement that might make illegal the types of activity — like espionage, data manipulation or attacks on infrastructure — that the United States may want to use in a future conflict.