ISLAMABAD, Pakistan — An unusually powerful American delegation arrived here on Thursday to deliver the starkest warning yet to Pakistan, according to a senior American official: that the United States would act unilaterally if necessary to attack extremist groups that use the country as a haven to kill Americans.

Secretary of State Hillary Rodham Clinton; David H. Petraeus, the new director of the Central Intelligence Agency; and Gen. Martin E. Dempsey, the new chairman of the Joint Chiefs of Staff, planned to push their Pakistani counterparts to make a definitive choice between fighting terrorists or supporting them, the administration official said.

“This is a time for clarity,” Mrs. Clinton declared in Kabul, Afghanistan, where she met President Hamid Karzai before leaving for Islamabad, the Pakistani capital. “No one should be in any way mistaken about allowing this to continue without paying a very big price.”

“There’s no place to go any longer,” Mrs. Clinton added, referring to Pakistan’s leaders, whom the administration has accused of equivocating by supporting the Afghan insurgency.