(Reuters) – The United States has presented evidence to Pakistan about the growing threat and reach of a militant faction which Washington suspects has ties to Pakistani intelligence, U.S. officials said on Wednesday.

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In the presentations, U.S. military leaders provided Pakistan’s army chief with information detailing the role of the Haqqani network in a string of increasingly brazen bombings, including one last month targeting the main NATO air base at Bagram in Afghanistan.

Washington has long pressed Islamabad to crack down on the Haqqanis in the North Waziristan tribal zone bordering Afghanistan, who are closely aligned with the Taliban, but U.S. officials acknowledge it is a hard sell because of resistance within Pakistani intelligence.

General David Petraeus, who oversees the Afghan war as head of U.S. Central Command, told a congressional hearing the Haqqanis had “transnational” ambitions, suggesting they could try to strike beyond Pakistan and Afghanistan.

Washington has issued similar warnings about the growing reach of the Pakistani Taliban, which investigators blame for a botched May 1 car bomb in New York’s Time Square.

There are strategic reasons for Pakistan’s hesitancy to attack the Haqqanis, a faction which some in Islamabad see as a strategic asset that will give them influence in any eventual settlement to the war in neighboring Afghanistan.

One U.S. official said “some elements” of Pakistani intelligence, but far from all, still support the Haqqanis.

Without mentioning the Haqqanis by name, Petraeus acknowledged long-standing ties between Islamabad and what he called “bad guys,” suggesting the relationships were useful to gather intelligence on the groups.

But he voiced confidence Pakistanis understood that “you cannot allow poisonous snakes to have a nest in your backyard, even if the tacid agreement is that they’re going to bite the neighbors kids instead of yours.”

“Eventually,” Petraeus said, “they turn around and bite you and your kids.”

Pakistan has denied a report by the London School of Economics that alleges enduring ties between its Inter-Services Intelligence (ISI) agency and the Afghan Taliban.

PAKISTAN INTELLIGENCE ROLE

The report said the agency not only funds and trains Taliban fighters in Afghanistan but is officially represented on the movement’s leadership council, giving it significant influence over operations.

Petraeus said there was “no question” Pakistan has maintained “a variety of relationships,” in some cases dating back decades, to groups which, with U.S. support, battled the Soviets when they occupied Afghanistan.

“Some of those ties continue in various forms, some of them, by the way, gathering intelligence,” he said.

“You have to have contact with bad guys to get intelligence on bad guys.”

Some of the groups in question, including the Haqqani network, are now leading the fight against Western forces.

The Pentagon has expressed confidence that Pakistan will eventually mount an offensive in North Waziristan, but has acknowledged the country’s armed forces were already stretched by operations in other tribal areas.

“The problem has been one of capacity. And again, we’re working hard to enable that capacity,” Petraeus said.

Petraeus, General Stanley McChrystal, the top U.S. and NATO commander in Afghanistan, and Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, discussed Haqqani’s alleged role in the bombings in a recent meeting with Pakistan’s army chief Ashfaq Kayani.

“We have shared information with him about links of the leadership of the Haqqani network … that clearly commanded and controlled the operation against Bagram air base and the attack in Kabul, among others,” Petraeus said.

Suicide bombers carrying rockets and grenades launched a brazen predawn attack on the base on May 19, killing an American contractor and wounding nine U.S. troops. About a dozen militants, many wearing suicide vests packed with explosives, were killed, the Pentagon said at the time.

A day earlier, a suicide bomber attacked a military convoy in Kabul, killing 12 Afghan civilians and six foreign troops.

Bagram is the main base for U.S.-led troops in Afghanistan, with the largest airfield in the country. It was used by the former Soviet Union during its invasion of Afghanistan in the 1980s.

(Additional reporting by Phil Stewart; editing by Todd Eastham)