WASHINGTON - A top secret cable from U.S. ambassador to Afghanistan has claimed that the presence of enemy safe havens in Pakistan could be a "deal breaker" in American efforts to end the country's longest war, according to a report published in The Washington Post Saturday.

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

The Post said several officials familiar with the cable's contents said it could be used as ammunition by senior military officials who favour more aggressive action by the United States against the Haqqani havens in Pakistan. It also could give support to calls from senior military officials for a more-gradual withdrawal of U.S. forces from Afghanistan as the 2014 deadline for ending combat operations approaches.

These military officials, according to the Post. have maintained for months that the current strategy of targeting raids against Taliban leadership and building local Afghan governance is showing impressive results. But they warn that worsening conditions in Pakistan and the ability of insurgent groups to find haven there necessitates a larger American force than many in the Obama administration are advocating.

The United States is on course to reduce the size of its force in Afghanistan to about 68,000 troops by the end of this summer and shift from combat to more of an advisory role to Afghan forces by the middle of next year.

The coming drawdowns will put heavy pressure on the Afghan government in the east, where U.S. and Afghan forces have struggled to curb violence, in part because insurgents can flee across the border to Pakistan, U.S. officials said.

“The sanctuaries are a deal-killer for the [Afghan war] strategy,” said a senior defence official who is familiar with the ongoing debate and who, like several officials in this story, spoke on the condition of anonymity to discuss sensitive internal deliberations.

In past years, U.S. military officials have argued that the best defence against Pakistan insurgent sanctuaries was a stronger Afghan army and government. But with U.S. drawdowns looming, the need to directly address the sanctuaries seems more urgent, they said.

The Haqqani network is responsible for some of the larger and more dramatic attacks on Kabul, including one on the U.S. Embassy last year, U.S. officials said.