Sen. John F. Kerry today called for an end to Bush administration programs aimed at developing a new generation of nuclear weapons, saying the United States must lead by example as it moves to curb proliferation and prevent terrorists from obtaining nuclear arms.

The Massachusetts senator, the Democratic Party's presumptive nominee to run against President Bush in November, said that, if elected, he would also work with Russia to accelerate reductions in U.S. and Russian nuclear arsenals and make it a top priority to halt the nuclear weapons programs of North Korea and Iran. He said he also would appoint a presidential coordinator to direct efforts to secure nuclear weapons and materials around the world and prevent a nuclear terrorist attack.

In a speech in West Palm Beach, Fla., Kerry called for new leadership "to prevent the world's deadliest weapons from falling into the world's most dangerous hands." He said, "If we secure all bomb-making materials, ensure that no new materials are produced for nuclear weapons and end nuclear weapons programs in hostile states like North Korea and Iran, we can and will dramatically reduce the possibility of nuclear terrorism."

Kerry accused the Bush administration of moving too slowly to safeguard nuclear materials around the world, notably in Russia, which he said was securing potential bomb material in the former Soviet Union at a pace that would take 13 years to complete.

"We cannot wait that long," Kerry said, according to a prepared text of his speech. He said he would ensure that the task is finished in four years.

Kerry, making the second of a series of campaign speeches on national security as part of an 11-day tour devoted to the issue, said the prospect that al Qaeda or other terrorist groups could obtain a nuclear weapon ranks as "the greatest threat we face today." Yet, despite the Sept. 11, 2001, terrorist attacks, the United States has not done enough to secure nuclear weapons and bomb-making materials, or to stop nuclear programs in North Korea and Iran.

In response to the speech, the campaign of President Bush and Vice President Cheney accused Kerry of playing politics with national security and ignoring the Bush administration's success in negotiations with Libya, which has agreed to dismantle its programs to develop nuclear arms and other weapons of mass destruction.

"John Kerry's embrace of nonproliferation goals and objectives already laid out by President Bush is a welcome step, but his failure to accept the success of negotiations with Libya and his criticism of a multilateral approach to confront the threat from North Korea demonstrate that John Kerry can't help but play politics with national security," said a statement issued by Steve Schmidt, a spokesman for the Bush-Cheney campaign.

The statement criticized Kerry's pledge to open bilateral talks with North Korea, in addition to an existing multilateral diplomatic process that also involves Russia, China, South Korea and Japan.

"Other than this immediate concession to North Korea, Kerry has offered no policy proposals for how to ensure that North Korea dismantles its nuclear programs," the statement said.

It made no mention of Kerry's vow to halt the development of new U.S. nuclear weapons.

In his outdoor speech at the port of Palm Beach, Kerry said the United States must be able to convince the world of its sincerity in leading an effort to reduce stockpiles of nuclear weapons and materials.

"As president, I will stop this administration's program to develop a whole new generation of bunker-busting nuclear bombs," Kerry said. "This is a weapon we don't need. And it undermines our credibility in persuading other nations. What kind of message does it send when we're asking other countries not to develop nuclear weapons, but developing new ones ourselves?"

Referring to what his campaign said was an effort to develop more "usable" nuclear arms such as "mini-nukes" and "bunker-busters," Kerry said, "We don't need a world with more usable nuclear weapons. We need a world where terrorists can't ever use one. That should be our focus in the post 9/11 world."

More than $27 million in research money for nuclear bunker-busters -- existing warheads with a new delivery system -- has been approved, and a defense authorization bill now before Congress includes nearly $500 million in additional funding to develop the weapon under a five-year plan. But production of the weapon cannot go ahead without congressional approval.

The Bush administration has sought a smaller amount of research funding -- $15 million to date -- to do studies on new weapons concepts. Among them are "mini-nukes" of less than five kilotons.

Kerry said that in addition to the current six-party talks with North Korea on its nuclear program, the United States "must also be prepared to talk directly with North Korea."

Declaring that "a nuclear-armed Iran is unacceptable," Kerry said the United States should organize other nations to offer the Iranians the nuclear fuel they say they need for a peaceful energy program in return for giving up spent fuel that can be diverted to weapons production.

"We should call their bluff," Kerry said. If Iran did not accept the deal, its true motivations would be clear, he said.

Staff writers Walter Pincus and Dan Balz contributed to this story.