The U.S. has helped five nations completely clear out their stocks of highly enriched uranium since President Barack Obama outlined his plans for securing all weapons-usable materials worldwide, officials say, citing it as progress in the administration's efforts to prevent nuclear weapons from getting in terrorists' hands.

Anne Harrington, the National Nuclear Security Administration's nonproliferation chief, said that since Obama's April 2009 speech in Prague announcing his plans, the U.S. has helped remove enough material from about a dozen countries to make almost 30 warheads. She added that several global leaders are expected to use a nuclear security summit in Seoul, South Korea, which starts Sunday, to announce similar advances.

Arms control experts say the most difficult part of building an atomic bomb is acquiring the weapons-grade uranium or plutonium needed for the explosive core of the weapon. Locking up or eliminating these materials is crucial to preventing nuclear-armed terror.

Harrington said that's the administration's top national security concern: ``Issue No. 1 ... above anything else, keeping this material out of the hands of terrorists,'' she said.

Over the past three years, officials say, the U.S. has helped Romania, Libya, Turkey, Chile and Serbia completely clear out their stockpiles of weapons-usable uranium. They join 13 other nations that did so previously _ Brazil, Bulgaria, Colombia, Denmark, Greece, Latvia, the Philippines, Portugal, Slovenia, South Korea, Spain, Sweden and Thailand.

For the most part, this has meant shutting down civilian research reactors fueled by weapons-grade uranium, or converting those reactors to use low-enriched uranium.

Progress hasn't always come easily. Secret State Department cables obtained by WikiLeaks show that in November 2009, Libyan officials threatened to renege on a pledge to allow the U.S. to fly Libya's remaining 5.2 kilograms (11.5 pounds) of highly enriched uranium from its research reactor to Russia for storage and eventual destruction.

After delays and negotiations, Libya relented and the uranium was loaded onto a Russian cargo plane in late December of that year.

The shipment occurred a little over a year before the start of the uprising that ousted Moammar Gadhafi, which could have wrecked the deal and, perhaps, left Libya's nuclear material up for grabs. Its departure completed the dismantlement of Libya's atomic weapons program.

In August last year, Belarus canceled an agreement to surrender hundreds of pounds of weapons-usable uranium after the U.S. imposed sanctions in response to President Alexander Lukashenko's crackdown on political opposition.

In November 2007, South Africa witnessed one of the most serious recent threats to nuclear material stockpiles, a raid on the Pelindaba nuclear site by two armed groups. But it has so far balked at giving up its stockpile of weapons-usable material, the largest of any developing nation, arguing that the U.S. hasn't done more to reduce its own inventory.

Obama has made reducing the global stockpile of nuclear weapons and materials a key goal of his foreign policy. He has pledged to work toward a world free of nuclear weapons, pushed through a New START treaty that trimmed the U.S. and Russian nuclear arsenals and is currently weighing options for sharp new cuts to the U.S. nuclear force, including a reduction of up to 80 percent in the number of deployed weapons.

The Associated Press reported in February that the administration is considering at least three options for cutting deployed strategic warheads to around 1,000-1,100, 700-800 or 300-400.

Countries known to have nuclear weapons are the U.S., Russia, Britain, France, China, Israel, India, Pakistan and North Korea. Scores of countries still have research reactors fueled by weapons-usable uranium, and medical devices that use radioactive materials that could be fashioned into a ``dirty bomb'' are scattered all over the world.

An Arms Control Association study found overall that global leaders kept about 80 percent of 67 commitments, made at a 2010 nuclear security summit in Washington, to reduce or better secure their stocks of nuclear materials. But the study cautioned that ``the nuclear material security challenge will not be solved even after all the national commitments made ... are completed.''

Gary Samore, coordinator for arms control with the National Security Council, told reporters Wednesday that the 80 percent figure was ``a very good batting average.'' During the upcoming summit, he said, the U.S. would press for further commitments, this time with an emphasis on preventing the smuggling of nuclear materials and secrets.

Nuclear nonproliferation expert Matthew Bunn at Harvard University said he would not rule out some kind of settlement of the U.S. dispute with Belarus during the Seoul summit, adding that a number of nations with small stockpiles of weapons-grade material may also announce they have cleaned them out.

Meanwhile, he said, several countries could commit themselves to ratifying relevant treaties, hosting International Atomic Energy Agency reviews of their security arrangements or creating law enforcement teams to indict nuclear smugglers. But Bunn said he expects that much more will remain to be done.

``It will, I believe, be possible to say at the end of four years that for most or all of the highest-risk stocks, some significant progress has been made and the risks have been reduced,'' said Bunn, who advised the Clinton administration on efforts to lock up the former Soviet Union's vast stores of nuclear materials. ``What it will not be possible to say at the end of four years is that all the nuclear weapons and weapons-usable material is secured and accounted for.'' (AP)