WASHINGTON -- Preparing for a July summit on the issue with his Russian counterpart, President Barack Obama on Tuesday said it is "absolutely imperative" that the United States take the lead in reducing the spread of nuclear weapons. 

Obama's vision for a world free of such weapons was the subject of a major speech he delivered last month in Prague. The topic also will be on the agenda for his coming meetings with Russian President Dmitry Medvedev in Moscow. 

Obama said the U.S. must take the lead, particularly at a time when North Korea and Iran are developing nuclear weapons capabilities, nuclear-armed Pakistan and India remain in a long-running conflict, and terrorist groups like al-Qaida are trying to get such weapons. 

"It is absolutely imperative that America takes leadership working with, not just our Russian counterparts, but countries all around the world to reduce and ultimately eliminate the dangers that are posed by nuclear weapons," the president said after an Oval Office meeting with former secretaries of state George P. Shultz and Henry Kissinger, former Defense Secretary William Perry and former Sen. Sam Nunn of Georgia. 

Obama mentioned possible steps to "lock down loose nuclear weapons," including revitalizing the Nuclear Nonproliferation Treaty, making progress on a Comprehensive Test Ban Treaty and working with Russia to reduce dependence on nuclear weapons. 

Russia and the U.S. have the world's largest nuclear stockpiles. 

"We are going to be pushing this as one of our highest priorities, to take specific steps, measurable steps, verifiable steps, to make progress on this issue, even as we keep a long-term perspective and a long-term vision about what can be achieved," Obama told reporters. "And we can think of no better advisers, counselors, and partners in this process than the four gentlemen who joined us here today." 

Speaking for the group, Shultz said they "support enthusiastically what the president is doing."

But Shultz, who was Ronald Reagan's secretary of state and supported Republican John McCain over Obama in last year's White House campaign, disagreed with Obama's description of the meeting as a reminder "of the long tradition of bipartisan foreign policy." 

"You said that you welcome the fact that this is bipartisan. And, well, it is," said Shultz, sitting to Obama's right. "At the same time, I think all of us have said, when people have told that to us, that it's really nonpartisan. This is a subject that ought to somehow get up above trying to get a partisan advantage."