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.

The cable, which was described by several officials familiar with its contents, could be used as ammunition by senior military officials who favor more aggressive action by the United States against the Haqqani havens in Pakistan. It also could buttress 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 have maintained for months that the 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 American frustration with insurgent sanctuaries in Pakistan has long been a source of tension in the brittle relations between the two countries.

“The sanctuaries are a deal-killer for the [Afghan war] strategy,” said a senior defense 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 defense against insurgent sanctuaries in Pakistan was a stronger Afghan army and government. But with U.S. drawdowns looming, the need to directly address the sanctuaries seems more urgent.

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.

The group’s patriarch, Jalaluddin Haqqani, was a major mujaheddin fighter in the CIA-backed effort to expel the Soviets from Afghanistan in the 1980s. He has relinquished control to his son, Sirajuddin, who carries a $5 million U.S. bounty on his head and runs day-to-day operations from the network’s Pakistani base in Miran Shah.

The location has given the Haqqani leadership a measure of protection. The CIA has repeatedly refrained from launching missiles at known Haqqani targets, including a prominent religious school the network uses as a base of operations, out of concern for civilian casualties and the backlash that could ensue.

A stark message

The U.S. Embassy in Kabul declined to comment on Crocker’s cable. “As a policy, we don’t comment as to the existence or substance of top-secret cables,” an embassy spokesperson said.

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

Crocker previously served as U.S. ambassador to Pakistan during the George W. Bush administration and was brought out of retirement by President Obama. Crocker also built close ties to the military and to David H. Petraeus, now CIA director, when Crocker was the ambassador to Iraq and Petraeus was the top general there.

As commander of U.S. and NATO forces in Afghanistan, Petraeus frequently voiced deep concern about the Haqqani group’s resilience.

The CIA has carried out dozens of drone strikes against Haqqani targets in Pakistan in recent years, while U.S. forces on the Afghan side of the border have killed or captured Haqqani fighters at a rapid pace — only to see the flow of militants subside temporarily and then resume.

“There’s no debate about the importance of going after Haqqani . . . and Taliban militants who launch attacks into Afghanistan,” one U.S. official said. “Support for this is universal.”

Repeated vows to escalate the U.S. campaign against the Haqqani network have produced seemingly fleeting results. 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 killed Janbaz Zadran, described by CIA analysts as the main organizer of attacks against coalition targets in Kabul and southeast Afghanistan.

But the timing of Crocker’s cable — sent more than two months after that CIA strike — suggests that U.S. officials in Kabul have yet to see a shift in momentum or measurable impact. The U.S. 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.

Unusual mode of delivery

The somewhat unusual mode of transmission for Crocker’s cable suggests that its contents were particularly sensitive, U.S. officials said.

American ambassadors typically send messages to Washington through State Department communications networks. But U.S. officials said cables containing references to intelligence sources or highly classified threat data can be sent across CIA networks, which are more secure. The CIA declined to comment on the cable.

Some current and former U.S. officials have questioned whether the Haqqani network presents an existential threat to the Afghan government.

The Haqqani network’s area of operation in Afghanistan is limited primarily to a handful of provinces in the east, along the border with Pakistan. Some U.S. officials have said the Afghan government’s corruption and its inability to provide services to its people pose a greater threat to the success of the U.S. mission in Afghanistan than the Haqqani network or even the Taliban.

Network seen as a proxy

CIA strikes in Pakistan were suspended for weeks last year after the arrest of an agency contractor on charges of killing two Pakistani men in Lahore and, later, after the U.S. strafing of a Pakistani border post in November that left two dozen of the country’s soldiers dead.

Pakistan has been unwilling to use its military against the Haqqani network, which is seen by many as a Pakistani proxy that is careful to avoid provoking its host, instead directing its attacks against American, Indian and Afghan targets.

Though secret, Crocker’s cable was the latest expression of American exasperation with the situation.

In September, Adm. Mike Mullen, then the chairman of the Joint Chiefs of Staff, vented publicly, testifying before Congress that the Haqqani network was a “veritable arm” of Pakistan’s Inter-Services Intelligence agency.

U.S. officials subsequently said Mullen’s characterization overstated the relationship, but many remain convinced that the network couldn’t operate without tacit support from Pakistan.