WASHINGTON (Reuters) - Nuclear terrorism is the gravest threat the United States faces, Democratic presidential candidate John Kerry said on Tuesday as he offered a plan to secure atomic arsenals and materials around the world.

"The enemy is different and we must think and act anew," Kerry said in excerpts of remarks prepared for delivery in West Palm Beach, Florida. "We have to do everything we can to stop a nuclear weapon from ever reaching our shore and that mission begins far away."

In the second of three speeches on national security, Kerry is expected to propose a new high-level White House job to oversee efforts to prevent a terrorist attack using nuclear weapons and recommend speeding up a current program to secure nuclear material in the former Soviet Union.

"The greatest threat we face today (is) the possibility of al Qaeda or other terrorists getting their hands on a nuclear weapon," Kerry said. "Osama bin Laden has called obtaining a weapon of mass destruction a 'sacred duty."'

The senator from Massachusetts, a 20-year veteran of the Foreign Relations Committee, also said he wanted to end nuclear programs in countries like Iran and North Korea.

Kerry has criticized President Bush for refusing to hold bilateral negotiations with North Korea. He has said he would adopt a two-track policy of continuing the six-party talks that include Russia, Japan, China and South Korea while also holding direct discussions with Pyongyang.

After the Sept. 11, 2001 attacks on the United States, Kerry said Americans needed to "take away politics, strip away the labels" and ask honest questions.

"Have we done everything we could to secure these dangerous weapons and materials? Have we taken every step we should to stop North Korea and Iran's nuclear programs? Have we reached out to our allies and forged an urgent global effort to ensure that nuclear weapons and materials are secured?"

NUCLEAR DEACTIVATION

"The honest answer, in each of these areas, is that we have done too little, often too late, and even cut back our efforts," he said.

A Kerry foreign policy adviser said when Bush came to office he curtailed the Cooperative Threat Reduction Program, known as Nunn-Lugar after the two senators -- Democrat Sam Nunn and Republican Richard Lugar -- who created it.

In the past decade, the program has spent $4 billion to help former Soviet states eliminate or secure nuclear, biological and chemical weapons, including the deactivation of more than 6,000 nuclear warheads.

Kerry has supported expanding and accelerating Nunn-Lugar as an important defense against terrorists and rogue states obtaining old Soviet weapons of mass destruction.

"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 will dramatically reduce the possibility of nuclear terrorism," he said.

The presumptive Democratic nominee, who is locked in a tight battle with Bush five months ahead of the Nov. 2 election, has launched an 11-day mini-campaign devoted to national security as the chaos in Iraq and the June 30 handover to an as-yet-unnamed interim government dominates the headlines.

Last week, he outlined four "imperatives" -- rebuilding alliances "shredded" by Bush's go-it-alone policies, modernizing the U.S. military, using diplomacy, intelligence, economic power and American values to defeat threats and freeing the United States from its "dangerous dependence on Middle East oil.