Obama spoke alongside Pakistani Prime Minister Yousuf Raza Gilani before the two men held private talks. The meeting comes amid heightened tensions between their countries following a series of incidents that have marred trust.
`'There have been times -- I think we should be frank -- in the last several months where those relations have experienced strains," Obama said of the dynamic between the U.S. and Pakistan.
The breakdown in the relationship followed the shooting death of two alleged Pakistani assailants by a CIA officer and the U.S. Navy SEAL raid that killed Osama bin Laden in May. The tipping point came in November, when U.S. forces returned fire they believed came from a Pakistani border post, killing 24 Pakistani troops.
Pakistan broke off ties with the U.S. following that incident and launched a debate about new terms of engagement with the U.S., including on the sensitive issue of CIA drone strikes on targets inside Pakistani borders.
Pakistan has rejected offers by U.S. officials to give its spy chief advanced notice of the CIA's drone campaign against al-Qaida in Pakistan and to apply new limits on the types of targets hit.
Obama said it was important for the U.S. and Pakistan to have candid and open talks. He said he expects Pakistan's review will result in a "balanced approach that respects Pakistan's sovereignty but also respects our concerns with respect to our national security and our needs to battle terrorists who have targeted us in the past."
Pakistan is a key U.S. counterterrorism partner and its cooperation is essential for drawing down the American-led war in neighboring Afghanistan.
Gilani, speaking at the start of his meeting with Obama, said he wanted stability in Pakistan and Afghanistan. And he said he appreciated Obama's statements of respect for Pakistan's sovereignty.
Obama and Gilani were meeting on the sidelines of an international nuclear security summit in the South Korean capital.
During remarks at the summit earlier Tuesday, Obama said the threat of nuclear weapons remains a potent challenge for the globe to confront, telling the more than 50 world leaders attending the meetings that "the security of the world depends on the actions that we take."
Obama said the international community had made progress in removing nuclear materials and improving security at nuclear facilities around the globe. But he warned "there are still too many bad actors in search of these dangerous materials and these dangerous materials are still vulnerable in too many places."
Action at the summit has been largely overshadowed by competing forces, including North Korea's plan for a long-range rocket launch and Obama's caught-on-tape remarks plotting strategy with Russia after the November elections.
Obama himself brought North Korea into the discussions in Seoul, using nearly all of his appearances during his three-day trip to warn Pyongyang that it would face further isolation if it proceeds with the launch.
The president also found himself trying to clean up a controversy created when an exchange with Russian President Dmitry Medvedev that was meant to be private was picked up by a microphone. Obama was heard saying he would have more room to negotiate on missile defense after getting through the November election, presumably expecting to win and not have to face voters again.
Obama's Republican rivals back home pounced, accusing him of secretive plotting and dealing over American national security.
Obama tried to put the matter to rest on Tuesday, saying there was no way to expect progress on the contentious issue of missile defense during the politics of an election year in both the U.S. and Russia.