The decision to advance the departure was "made a while ago" after assessing the security situation, US National Security Adviser Stephen Hadley said.
He said security risks and precautions had been assessed prior to Bush's decision to go to Pakistan.
" at this point people are comfortable that the necessary precautions are in place but this is not a risk free undertaking," Hadley said. He will hold the talks with Pakistan President Pervez Musharraf on Saturday.
Bush will now fly to Islamabad on Friday evening after his address at Purana Qila and spend the night in the Pakistani capital.