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President Barack Obama, closing a nuclear security summit Tuesday, sought a thaw in the diplomatic chill with Pakistan, a critical but difficult U.S. partner whose nuclear weapons and historical links to terrorism make its arsenal among the world's most vulnerable.

"There have been times _ I think we should be frank _ in the last several months where those relations have experienced strains," Obama told Pakistani Prime Minister Yousuf Raza Gilani.

Their meeting broke a four-month moratorium on direct top level contacts between the United States and Pakistan. Obama and Gilani were among more than 50 leaders who met here to reaffirm controls on nuclear material that might be bought or stolen by terrorists for a bomb. Obama headed back to Washington after the summit ended.

Pakistan is a key U.S. counterterrorism partner and its cooperation is essential for drawing down the American-led war in neighboring Afghanistan. Pakistan's nuclear weapons, developed outside the international standards endorsed by the United States, are a principal reason the U.S. struggles to promote a stable and friendly government there.

Ties with Pakistan deteriorated last year after the military raid that killed al-Qaida leader Osama bin Laden, when the United States kept Pakistani officials in the dark about the operation. Relations ruptured further when U.S. forces killed 24 Pakistani forces in November in what the United States says was a case of mistaken identity. Pakistan broke off high-level 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.

Obama said the U.S. and Pakistan are seeking a balanced partnership 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."

Gilani said he was pleased by the reference to sovereignty. He did not address the sidelong reference to the bin Laden raid, which outraged Pakistanis more for its intrusion on Pakistani soil than for the revelation that the 9/11 mastermind was living comfortably outside the country's capital.

The U.S. is eager to resolve the Pakistan standoff and get Afghanistan war supplies moving through Pakistan again, because it has had to spend much more money shipping goods by an alternative route that runs through Central Asia.

The supply line through Pakistan will also be a critical necessity for trucking out equipment as the U.S. seeks to withdraw most of its combat forces from Afghanistan by the end of 2014.

Pakistan would also benefit from patching up relations because it needs U.S. assistance to help keep its struggling economy afloat. The U.S. has given Pakistan billions of dollars in aid since 2001 to enlist its support in fighting Islamist militants, but the relationship has been plagued by mistrust.

Obama's work in South Korea focused on threats posed by North Korea and Iran, and on the ongoing bloody government crackdown in Syria. Obama used the summit to hold separate meetings with leaders of China and Russia and Turkey, players in the Iran and Syria crises. With his South Korean hosts he was especially blunt in confronting North Korea over a planned long-range rocket launch, and vowed that "bad behavior" will not be rewarded with negotiations or aid.

Obama claimed 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."

U.S. intelligence agencies rate the risk of nuclear proliferation and nuclear terrorism high on the list of global risks _ just below threats posed by Iran and al-Qaida.

Pakistan's arsenal has caused concern for intelligence agencies and arms control experts for years. A 2010 Harvard study found that Pakistan's arsenal "faces a greater threat from Islamic extremists seeking nuclear weapons than any other nuclear stockpile on earth."

Alarm rose this year amid political upheaval that threatened the U.S.-backed civilian government.

Abdul Qadeer Khan, the architect of Pakistan's nuclear weapons program, has been accused of running a nuclear black market ring that sold weapons technology to Iran, North Korea and Libya. The ring operated for years before it was disrupted in 2003. Western experts say Pakistan has about 100 nuclear weapons and is in the midst of a rapid expansion of that arsenal.

There is some quiet cooperation between the two nations on securing those weapons. The U.S. has provided Islamabad with millions worth of aid to protect its weapons, including money for intrusion detection systems, advice on designing tiered defenses and training.

But U.S. officials say Islamabad has refused to give the U.S. access to sensitive sites or agree to any formal plan for joint action in an emergency.

Pakistan insists its nuclear arsenal is well-defended, and the widespread fear among many Pakistanis is that the main threat stems not from al-Qaida or the Taliban, but from suspected U.S. plans to seize the country's weapons.

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