A prominent U.S. newspaper says the U.S. ambassador to Afghanistan sent a top-secret cable to Washington last month warning that the existence of “enemy havens” in Pakistan was placing the success of the U.S. strategy in Afghanistan in jeopardy. The report says the message was so secretive that it was sent through CIA channels rather than the usual ones in the State Department.

The Washington Post, citing unnamed officials, reported late Friday that Ryan Crocker wrote the cable that amounted to an admission that years of U.S. efforts to curtail militant activity in Pakistan by the Haqqani network were failing. The network is a key Taliban ally.

The report says one official described the Pakistan militant sanctuaries as a “deal-killer” for the Afghan war strategy.

The U.S. has long been frustrated with the insurgent havens in Pakistan. The relations between the two countries have further deteriorated since the U.S. raid last year in Pakistan that killed al-Qaida leader Osama bin Laden who was residing in a Pakistani garrison town.

The Washington Post says the contents of the cable could become “ammunition” for some senior military officials who favor more aggressive action by the U.S. against the Haqqani havens in Pakistan.

The report says the cable could also support calls by senior military officials for a more gradual withdrawal of U.S. forces from Afghanistan. The U.S. plans to end combat operations in 2014 and hand security over to the Afghans.

The U.S.-led coalition invaded Afghanistan in late 2001 to topple the hardline Islamists and to hunt for bin Laden after the September 11, 2001, attacks against the United States.