Secretary of State Hillary Clinton has warned Pakistan of unspecified "severe consequences" if it can be linked to a successful extremist attack on the US.
She told CBS while Pakistan had become more helpful in tackling extremists, co-operation could still be improved.
A Pakistani-born US citizen has been charged with the attempted bombing in Times Square in New York a week ago.
Earlier, Defence Secretary Robert Gates said the US was prepared to increase military assistance to Pakistan.
"We're willing to do as much... as they are willing to accept," he told reporters. "We are prepared to do training, and exercise with them. How big that operation becomes is really up to them."
But he played down the chances of an extended crackdown on militants, saying Pakistani forces were already "thinly stretched".
In an interview with CBS television's 60 Minutes programme, Mrs Clinton said there was now a "much better relationship" between the US and Pakistani governments, militaries and intelligence services.
"I think that there was a double game going on in the previous years, where we got a lot of lip-service but very little produced," she said.
But the past two years had seen "the killing or capturing of a great number of the leadership of significant terrorist groups", Mrs Clinton added.
"We've gotten more co-operation and it's been a real sea change in the commitment we've seen from the Pakistan government. We want more. We expect more."
"We've made it very clear that if - heaven-forbid - an attack like this that we can trace back to Pakistan were to have been successful, there would be very severe consequences," she warned.
Pakistan's government has promised to co-operate with the investigation into the failed car-bomb attack in Times Square, which has uncovered possible links to the Pakistani Taliban and an Islamist group in Kashmir.
The main suspect, Faisal Shahzad, who was born in Pakistan and became a US citizen last year, has been charged with terrorism, attempting to use a weapon of mass destruction. He has yet to appear in court.
He is continuing to co-operate with investigators, and has admitted to receiving bomb-making training in the tribal region of Waziristan, prosecutors say.
ABC News has reported that Mr Shahzad told investigators he was angry because friends had been killed by CIA strikes in Pakistan, his personal life was in crisis, and he feared for the safety of his family.