In a 'top-secret' cable to the Obama administration, America's envoy to Kabul has warned that the existence of terror sanctuaries in Pakistan and insurgent activities of the 'lethal' Haqqani network were a 'deal-killer' for the US strategy in Afghanistan.

The cable written by Ambassador Ryan C. Crocker last month "amounted to an admission that years of US efforts to curtail insurgent activity in Pakistan by the lethal Haqqani network, a key Taliban ally, were failing," the Washington Post reported on Saturday.

"Because of the intended secrecy of that message, Crocker sent it through CIA channels rather than the usual State Department ones," it said.

"The sanctuaries are a deal-killer for the (Afghan war) strategy," a senior defence official, familiar with the ongoing debate, was quoted as saying by the daily.

The State Department refused to comment on the authenticity of the cable. "We are not in a position to comment on alleged leaked cables," its spokesman Mark Toner said when asked about the news report. "Our position on the Haqqani network remains as we've expressed it publicly.

Safe havens continue to pose a threat to Pakistan, Afghanistan and the United States and we must do all we can together to see them eliminated," Toner told agencies.

The Washington Post, however, had not been able to get into the specific details of the cable sent by Croker, except for saying that its content was sensitive that he decided to send it through the CIA channels, which has a more secure communication network.

"The cable has drawn attention in Washington because of its stark message and because American ambassadors rarely argue that the US government must take more forceful action in another country. Officials familiar with the cable declined to name its primary recipient," the daily said.

It continued to refer to the well-known allegation – that has been doing the rounds for the past several months in Washington now that Pakistan continues to support the Haqqani network.

"There's no debate about the importance of going after Haqqani ... and Taliban militants who launch attacks into Afghanistan — support for this is universal," one US official was quoted as saying.

Repeated vows to escalate the US campaign against the Haqqani network have produced seemingly fleeting results, the report said.

A CIA drone strike in October was described at the time by Obama administration officials as the opening salvo in a more aggressive assault against the group's leadership in Pakistan.

The missile attack had killed Janbaz Zadran, described by CIA analysts as the main organiser of attacks against coalition targets in Kabul and southeast Afghanistan, the report added.

"But the timing of Crocker's cable — which was sent more than two months after that CIA strike — suggests that US officials in Kabul have yet to see a shift in momentum or measurable impact."

"The US efforts have been hampered by the group's populated sanctuary, its close ties to Pakistan's intelligence service, and diplomatic ruptures that caused pauses in the CIA drone campaign," the daily reported.