Under the headline "After 'Decline,' U.S. Again Capable of Making Nuclear Arms," the Los Angeles Times, which broke the story Wednesday, called the move "an important symbolic and operational milestone in rebuilding the nation's nuclear weapons complex."

Specifically, American scientists working for the National Nuclear Security Administration, or NNSA, have started producing the plutonium "pits" that are at the core of nuclear weaponry. (Conventional explosives encase a hollow plutonium sphere, or pit, and trigger a chain reaction when detonated.)

Under a program put forward by the White House, the United States is also working on a new factory to supply components for hundreds of weapons each year, according to the report.

The U.S. Department of Energy, which oversees the NNSA and runs America's weapons program, could not be reached for comment late Wednesday. But the Times quoted unnamed department officials as denying that they are actually producing nuclear weapons -- only ensuring the reliability of exiting weapons.

But nuclear scientists in both Russia and the United States disputed this claim.

"Pits are empty spheres of plutonium, they cannot age," said a senior nuclear expert at one of Russia's leading institutes, who spoke on condition of anonymity.

"Such production cannot be justified by the need to maintain the safety of the existing stockpile of U.S. weapons. First of all, it could mean that America has restarted the production of nuclear warheads and that it is supporting the industry," the expert said.

"In Russia, such workshops are being closed down."

Arjun Makhijani, an acclaimed nuclear scientist who runs the Institute for Energy and Environmental Research in Tacoma, Washington, agreed: "There is absolutely no need in my opinion to do this. On the contrary, it is very dangerous," Makhijani said by telephone.

"This is just the beginning of pit manufacturing. The U.S. has a capacity to eventually make 50 to 80 pits a year, but the Department of Energy has proposed to build a new pit facility where they will be able to make up to 500 pits per year. The United States does not need any more nuclear warheads."

Igor Ostretsov, the deputy director for science of the All-Russia Research Center of Nuclear Machine-Building, said that while the United States may need new parts to maintain the efficiency of its warheads, it looks as if it is also moving to improve its nuclear arsenal.

"If they are making pits, it may be linked to making new [nuclear warhead] models," he said.

The move may also violate the Nonproliferation Treaty that the United States, Russia and other nuclear nations signed in 2000, in which they pledged to undertake an "irreversible reduction" of their nuclear arsenals.

Under Article 2 of the treaty, signatories are forbidden from manufacturing or otherwise acquiring nuclear weapons or other nuclear explosive devices.

"I don't know whether it will reignite the arms race, but it is certainly in line with the U.S. strategy of continuing to use nuclear weapons as a central part of its military strategy," Makhijani said.

Some military experts also said that the real aim of the program appears to be boosting the United States' nuclear complex -- a costly move that makes no strategic sense.

"It is a sign that after a long period of decline, the weapons complex is back and growing," Jon Wolfsthal, deputy director of the Carnegie Endowment for International Peace and a former Energy Department weapons expert, told the Times.

"To the average U.S. citizen, it would be accurate to say we have restarted the production of nuclear weapons."

Ivan Safranchuk, a Moscow-based researcher for the Center for Defense Information in Washington, said by telephone that it would be senseless militarily for the United States to improve its nuclear warhead arsenal, "which is excessive anyway and is supposed to be reduced."

Makhijani said "U.S. policy is a provocation to proliferation because it raises the question that if the most powerful country in the world by far, in conventional, or non-nuclear terms, still needs to build more nuclear weapons, what about everybody else?

"It is a dangerous policy because the United States and Russia continue to have between them about 4,000 nuclear weapons that can be fired in a few minutes."