ISLAMABAD, Pakistan — The Obama administration has delivered new and stiff warnings to Pakistan after the failed Times Square car bombing that it must urgently move against the nexus of Islamic militancy in the country’s lawless tribal regions, American and Pakistani officials said.

The American military commander in Afghanistan, Gen. Stanley A. McChrystal, met with the Pakistani military chief, Gen. Ashfaq Parvez Kayani, at his headquarters here on Friday and urged Pakistan to move more quickly in beginning a military offensive against the Pakistani Taliban and Al Qaeda in North Waziristan, Americans and Pakistanis familiar with the visit said. The officials spoke on the condition of anonymity because of the delicacy of continuing diplomatic efforts here.

The Pakistani-American man who admitted to the Times Square attack, Faisal Shahzad, 30, told American investigators that he had received training in North Waziristan, the main base for the Pakistani Taliban, Al Qaeda and other militant groups.

The new pressure from Washington was characterized by both the Pakistani and American officials as a sharp turnaround from the relatively polite encouragement adopted by the Obama administration in recent months. And it comes amid increasing debate within the administration about how to expand the American military’s influence — and even a boots-on-the-ground presence — on Pakistani soil.

Though the bombing in Times Square failed, Mr. Shahzad’s ability to move back and forth between the United States and Pakistan has heightened fears in the Obama administration that another attempt at a terrorist attack could succeed.

“We are saying, ‘Sorry, if there is a successful attack, we will have to act’ ” within Pakistan, one of the American officials said.

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