WASHINGTON—Intelligence threats against the U.S. are becoming more complex, diverse and harmful as adversaries turn to innovative hybrid techniques combining traditional spying, economic espionage and cyber operations to steal U.S. secrets, according to a government report released Monday.

The report, outlining a national counterintelligence strategy, said the availability of technologies such as advanced encryption and big-data analytics has made it easier for foreign intelligence services, as well as so-called hacktivists...