The U.S. ambassador to Afghanistan sent a top-secret cable to Washington last month warning that the persistence of enemy havens in Pakistan was placing the success of the U.S. strategy in Afghanistan in jeopardy, U.S. officials said.

The cable, written by Ryan C. Crocker, amounted to an admission that years of U.S. efforts to curtail insurgent activity in Pakistan by the lethal Haqqani network, a key Taliban ally, were failing. Because of the intended secrecy of that message, Crocker sent it through CIA channels rather than the usual State Department ones.