But the next head of the U.S. Senate's powerful foreign relations committee also suggested that generous American assistance to Pakistan depended on it capping past support for extremist groups.
India is suspicious of Pakistan's avowed clampdown on Lashkar-e-Taiba, the militant group New Delhi blames for last month's attack, which killed more than 160 people and raised concern that the nuclear-armed neighbors could slide into their fourth war.
Pakistani authorities have raided suspected militant training camps and confirmed the detention of three key suspects, including two alleged plotters of the attacks and the purported head of Lashkar-e-Taiba.
Kerry said he had learned during two days of talks with Pakistani government and military leaders that authorities had taken more action than had been announced.
"I know that more people have been detained than India is aware of," Kerry told reporters in Islamabad on Tuesday.
He didn't name any of the additional detainees, but said Pakistani authorities were actively "building the case" against those seized so that they can be prosecuted.
Lashkar-e-Taiba is one of a host of violent extremist groups that emerged after the U.S.-funded guerrilla war against Soviet forces occupying Afghanistan in the 1980s and opened a new holy war against Indian troops in Kashmir.
However, the group's operations have continued, fueling the suspicions of India and many Western observers that it still enjoyed covert support from Pakistani intelligence agencies.
Kerry said it was "a fact" that Pakistan's Inter-Services Intelligence was involved in creating the group, which has at times fought openly alongside Pakistani soldiers in Kashmir.
In New Delhi over the weekend, Kerry called for a reformed ISI, currently led by an army general, to be placed under close civilian control.
Some U.S. officials have supported Indian claims that the ISI was involved in the bombing of the Indian Embassy in the Afghan capital in July of this year.
Pakistani leaders complain that Indian investigators are not sharing evidence from Mumbai, raising concern that, as after the 2001 parliament attack, detained militant leaders will eventually be released without charge.
However, Kerry expressed optimism that Pakistan's leaders realized that Lashkar-e-Taiba had "morphed into a more al-Qaida-esque and radicalized entity" that had run out of control and represented a serious threat to Pakistan.
Kerry said he was struck by the determination of President Asif Ali Zardari and army chief Ashfaq Parvez Kayani to "crack down on any entity that tries to sidetrack the country from its democratic path and pursue the road of terror."
"I believe they are sincere," Kerry said.
He said Pakistan had made a choice that it is "on the side of civilized behavior and democracy."
"We're not going to provide help without a firm understanding of the rules of the road" in dealing with militants, Kerry said. "The time has come to put that choice to the test and I think the (Pakistani) government has understood that."