ISLAMABAD, Pakistan – Pakistan's closure of a border crossing to NATO supply convoys after a deadly air strike Thursday sent a blunt signal to Washington not to overstep as it looks for a new strategy to deal with Afghan insurgents that operate from Pakistani territory.

It also reflected a growing rift between the two governments prompted by Pakistan's unwillingness to move against insurgents strongholds in North Waziristan, which has led U.S. commanders in Afghanistan to adopt a more aggressive stance about attacking across the border when it has clear evidence that insurgents are moving back and forth, senior U.S. military officials in Afghanistan said.

The CIA also has stepped up drone strikes in North Waziristan over the past month.

The Pakistani move to close Torkham Gate, a narrow border crossing into Afghanistan through which hundreds of trucks carrying NATO supplies move every week, appeared unlikely to cause a permanent rift in the relationship between Islamabad and Washington.

"We hope the Torkham closure is temporary and are talking with the Pakistanis to resolve the issue," said Col. Dave Lapan, a Pentagon spokesman. "Transit through Pakistan is something we are very appreciative of and shows that we are partners, as we will continue to be."

Closing the gate to the convoys came after Pakistani military officials said two NATO helicopters crossed over into Pakistan's Kurram tribal region along the Afghan border before dawn Thursday and fired on Pakistani troops at the Mandata Kandaho border patrol post.

When the soldiers fired back at the helicopters, the aircraft retaliated by firing two missiles, destroying the post and killing three soldiers, the Pakistani military said. Within hours, Pakistani authorities ordered a halt to all trucks and oil tankers ferrying supplies to NATO forces through the Torkham checkpoint at the Afghan border.

The Pakistani military said the location of the check post, about 650 feet inside its border, was known to NATO. U.S. officials said the NATO helicopters were pursuing insurgents near the border and only returned fire after they were fired upon.

The incident and other recent strikes in Pakistani territory reflects a growing U.S. willingness to take unilateral action, even if it draws Islamabad's ire. How far the U.S. is willing to take this new effort is unclear and several officials said they expected the row to ease in coming days.

But one military officer said the U.S. might continue to carry out occasional helicopter attacks across the border into Pakistan, along with a stepped-up campaign of drone strikes, because senior U.S. commanders have decided that Pakistan does not intend to move against Afghan insurgents groups.

Though such attacks are little more than pinpricks, they keep pressure on insurgents infiltrating back and forth across the border, said the officer, who asked not to be named due to the sensitivity of the matter.

Though Pakistan has long tolerated and even assisted the CIA drone campaign on its territory, the more overt U.S. military moves in recent days have upset the understanding between the two governments that such activity must be kept, if not secret, then at least unacknowledged.

The Pakistani reaction to the latest incident, however, has been particularly vehement. Interior Minister Rehman Malik called for a full investigation into the deaths of the Pakistani troops "to determine whether this was deliberate or by mistake…. We will have to see whether we are allies or enemies."

U.S. officials played down the closing of the gate and said they expected the relationship to quickly return to normal.

"We have multiple routes to be able to resupply our forces in Afghanistan," said State Department spokesman P.J. Crowley.

The recent NATO strikes in Pakistani territory could reflect a new resolve by Western forces to fight back when Taliban militants launch attacks into Afghanistan from the relative safety of their bases in Pakistan's tribal areas.

The risk in that strategy, say Pakistani security analysts, is the damage it might do to Pakistan's willingness to cooperate.

"I think Pakistan has to reconsider this whole war on terror, and I think this thinking is already underway," said Mahmood Shah, a Peshawar-based defense analyst. "We should consider this to be deliberate policy on the part of the U.S."

david.cloud@latimes.com

alex.rodriguez@latimes.com

laura.king@latimes.com

Cloud reported from Washington, Rodriguez from Islamabad and King from Kandahar.