WASHINGTON (Reuters) - Washington’s push on Pakistan to get tough on militants on its territory is prompted by worries about an attack on U.S. soil, a concern the United States will press in talks with Islamabad later this month.

Residents on a horse-led cart past burnt fuel tankers meant for NATO troops, which was set on fire by gunmen on Thursday, along GT road in Nowshera in Pakistan's Khyber-Pakhtunkhwa Province October 10, 2010. REUTERS/Fayaz Aziz

A U.S. official last week countered suggestions that the tougher approach is driven by the need to show progress ahead of the October 22 talks by an Obama administration strategy review of the war in Afghanistan in December.

The failed Times Square bombing in May and the recent terrorism alert for Europe fueled fears of an attack, prompting the stepped up drone attacks in Pakistan’s rugged northwest and pointed U.S. comments pressing Islamabad’s to pursue militants more aggressively.

“There is really mounting concern that we are extremely vulnerable to an attack from a group in Pakistan that could occur,” the U.S. official said, speaking on condition of anonymity.

Such an attack would trigger a critical change in ties with Islamabad, the official warned.

“(An attack) will change the nature of the relationship, not because necessarily it makes sense to, but because the congressional outcry and the public outcry will be such that you will have to dramatically do things quite differently,” the official added.

Blunt words in Washington about Pakistan’s failure to aggressively go after insurgents coincided with a cross-border incursion by U.S.-led NATO forces that killed two Pakistani frontier guards and wounded several others. The incident ignited public outrage and prompted officials to close a key border crossing to NATO supply convoys for days.

While Pakistanis burned trucks carrying war goods, U.S. officials publicly apologized to Islamabad for the incursion on one hand while holding their ground on the need for the Pakistani military to deal aggressively with insurgent groups.

“We have a very difficult and complicated situation in Pakistan. We have worked hard on this relationship. We understand it’s important to our security,” White House spokesman Robert Gibbs said last week.

TALKS COMING UP

The latest tensions come just ahead of the U.S.-Pakistani Strategic Dialogue later this month, and the Washington talks would go more smoothly if the strains of the last two weeks were patched up by then.

“I think it’s going to be critical in the next few weeks as they prepare for the strategic dialogue between Pakistan and the U.S. ... that they don’t have any of these flare-ups, neither do they have any lingering doubts about each other’s intentions,” said Shuja Nawaz, director of the South Asia Center at the Atlantic Council, a Washington think tank.

That will require a lot of “direct communication,” he added, and not dealing with each other via the media.

The administration official agreed that “ratcheting up pressure on Pakistan ... is not the way to get them to do things.”

“With Pakistan, as with most countries, the more public and vocal you are, the harder things get,” he said.

But the United States will continue to deliver the message that Pakistan needs to do more against insurgent groups, even at the strategic review.

“It doesn’t need to be confrontational,” the official said. “We will continue with this message,” he said. “We have constant communications with the Pakistanis. They know where we’re at.”

U.S.-Pakistani relations are dogged by suspicion and mistrust. Although it has repeatedly engaged with Pakistan over the years, the United States has just as frequently cut off aid and disengaged, to punish Islamabad for actions like pursuit of nuclear weapons or through disinterest.

“It’s been like a roller coaster going back nearly 60 years,” said Teresita Schaffer, head of the South Asia program at the Center for Strategic and International Studies.

AFGHANISTAN TIMETABLE

Some analysts said the most recent flare-up appeared to be driven by the U.S. need to show some progress ahead of President Barack Obama’s December strategy review for the Afghanistan war, which is expected to set the stage for the start of a U.S. transition out of the country in July 2011.

“What seems to be driving all this is the shortening U.S. timetable for beginning the transition out of Afghanistan and the need to create some military momentum and space,” Nawaz said.

Unless Gen. David Petraeus, the top NATO commander in Afghanistan, can show significant movement toward U.S. objectives by July 2011, he will have difficulty justifying maintaining 100,000 U.S. troops in the country, a congressional aide said on condition of anonymity.

A congressionally mandated White House report sent to Capitol Hill this week assessing the war strategy said Pakistan had failed to move aggressively against al Qaeda and the Afghan Taliban last spring, in part for political reasons.

The Obama administration official denied any “Machiavellian plan” to put pressure on Islamabad to move more aggressively ahead of the U.S.-Pakistan Strategic Dialogue or Obama’s December strategy review.

“The report was due when it was due,” the official said, calling it a candid assessment to help Congress understand the challenges. The remarks highlighted in news accounts were more negative than the overall report, including classified sections that remain secret, he said.

“We were trying to tell Congress that the situation in Pakistan is rather dire and that we’re concerned,” the official said. “We’re worried that the day after tomorrow, there could be an attack that originates from Pakistan, from one of these groups, and that things will change dramatically.”