The United States yesterday secretly removed nearly 100 pounds of weapons-grade uranium from a research reactor in Poland, one of the largest recoveries yet of material that could be used to develop a nuclear bomb, government officials said.

Reflecting stepped-up efforts to curb the threat of nuclear terrorism, technicians from the US National Nuclear Security Agency recovered the highly enriched uranium from a civilian institute about 20 miles from Warsaw and transferred it, under heavy guard, to a facility in Russia.

There, the material will be "down-blended" into a safer form, according to the officials who spoke to the Globe but declined to be named until the operation is officially made public.

The two-day mission, secretly planned for months with the help of foreign authorities and the International Atomic Energy Agency, is part of an expanding US-Russian program, first launched two years ago, to secure nuclear and other radioactive materials at research facilities and other private locations across the former Soviet bloc and elsewhere.

It was the latest in a series of efforts in which US officials have taken control of under-protected weapons materials in a variety of places – ranging from Libya to Chechnya – where intelligence officials say they believe terrorists are seeking to obtain them.

But non-proliferation specialists contend that far more attention is needed to secure quickly vulnerable materials that remain at more than 160 research sites around the world.

And they express dismay that US intelligence agencies have yet to compile a comprehensive list of locations, ranked according to the dangerousness of the materials they contain and the degree of risk that they will be stolen by terrorists or black marketeers.

"This is an important development," said Anthony Weir , a research associate at Harvard University's Project on Managing the Atom. "But there is a long way yet to go. The important thing is to keep the eye on the ball of all the nuclear material, not just the material that is currently being dealt with. We need a comprehensive package for all the threats we are facing around the world."

The Poland operation was the latest to be undertaken as part of the Global Threat Reduction Initiative established in 2004.

In 14 shipments, more than 230 kilograms of highly enriched uranium has been returned to Russia under the program – from facilities in Serbia, Romania, Bulgaria, Libya, Uzbekistan, Latvia, and the Czech Republic.

The program has also recovered more than 2,700 radiological sources – such as used medical devices – within the United States and converted three research reactors from using highly enriched uranium to low-enriched uranium, which cannot be used for nuclear weapons. The effort is also responsible for installing new security enhancements at more than 400 sites around the world where vulnerable radiological materials are stored.

It has begun to give greater emphasis, however, to the vast network of research reactors that were established during the Cold War with both Soviet and American sponsorship and in many cases are in a state of disrepair or simply overlooked by local authorities.

Earlier this month, US officials oversaw the removal of more than 5,500 curries of radioactive cobalt-60 and cesium-137 – enough material for at least five radiological "dirty bombs" – from sites in the breakaway Russian territory of Chechnya.

Last month, 3 kilograms of highly enriched uranium was quietly removed from the Tajura reactor in Libya. Under heavy guard, it was loaded into three special containers and airlifted to Russia.

In four shipments between January and April, 63 kilograms – or 139 pounds – of nuclear fuel was removed from the Uzbekistan Institute of Nuclear Physics and shipped by rail to Tashkent, where it was then airlifted to a Russian facility.

The US government first organized the transfer of highly enriched uranium from the former Soviet Union in 1994, when in a military operation called "Project Sapphire," the US government returned nearly 600 kilograms of "fresh" HEU from Kazakhstan to Russia. In 1998, it recovered 5 kilograms of HEU from Tbilisi, Georgia.

But the effort has expanded dramatically in recent years.

"Before we were working on a pace of one every four years," said Laura Holgate , vice president for Russia programs at the non-profit Nuclear Threat Initiative in Washington.

The operation yesterday at the institute of Atomic Energy in Otwock-Swierk , Poland, was conducted with the aid of Polish authorities, the Russian government, and the International Atomic Energy Agency.

Under security provided by Polish police and special forces troops, the US agency removed the fresh HEU – a form of uranium before it is fed into the reactor – and it was loaded into five specialized containers by Department of Energy technicians and under the eye of the IAEA inspectors.

The operation was planned after US officials found out earlier this year about the previously unknown material stored at the institute. In the interim, the US officials paid for security upgrades at the site until the material could be removed. "It was material we did not know about," said a Department of Energy official involved in the operation yesterday.

Nonproliferation specialists, upon hearing of the operation, hailed the agency's efforts. But they all agreed that far more must be done to lock down the highly enriched uranium and other nuclear material that remains a deadly legacy of the Cold War.

"Are we doing more? Yes. Are we doing everything we need? No," said Joseph Cirincione, vice president for national security studies at the Center for American Progress in Washington. "It is as if we have all the time in the world. And we don't."