The new weapons could be used against deep bunkers
A leaked document suggests that Washington is beginning detailed planning for a new generation of smaller nuclear weapons.
The document - published by an anti-proliferation watchdog and confirmed as genuine by US officials - indicates the weapons could be used against targets like deep bunkers that contain chemical or biological agents.
The Los Alamos Study Group claims the plans would challenge the foundations of the Comprehensive Test Ban Treaty, which limits the development of new designs for nuclear bombs.
The BBC's Justin Webb in Washington says the plans clearly fit in with the wider Bush doctrine of pre-emptive strikes in the future when the US feels itself to be threatened.
The New Mexico-based Los Alamos Study Group posted on its website what it said were the minutes of last month's meeting in the Pentagon of senior US nuclear scientists.
It said the meeting was called to plan a secret conference "to discuss what new nuclear weapons to build, how they might be tested... and how to sell the ideas to Congress and the American public".
The group said that the conference of senior military officials and scientists would be held in August at the Omaha headquarters of the US Strategic Command in Nebraska.
The group did not say how it obtained the document, but said it decided to publish it taking into account Washington's "bold sweep of nuclear weapons planning".
"It's very rare that so many details about the nuclear weapons agenda of the Bush administration would appear in the same documents, in the same place," the group's spokesman, Greg Mello, said.
The minutes, which Bush administration officials confirm as genuine, also talk of lower yield nuclear weapons being developed with reduced collateral damage.
One of the principal tasks being considered for such devices is the destruction of deep bunkers where chemical, biological or nuclear weapons are stored.
President Bush has repeatedly stated that the US would consider pre-emptive strikes in the future if it considers itself to be threatened.