Both sides agreed to reduce operationally deployed strategic nuclear warheads to 1,700 from 2,200 by 2012.

A Multiple Reentry vehicle payload for a ballistic missile deploys multiple warheads in a pattern against a single target.

(As opposed to Multiple independently targetable reentry vehicle, which deploys multiple warheads against multiple targets.)

Evidently, Washington will also agree to a shared level of approximately 1,500-1,600 "deployed" warheads.

The replacement 2002 Strategic Offensive Reductions Treaty delayed reductions to 2012, with a limit of 2,200 operationally deployed warheads.

North Korea probably has fissile material for up to nine nuclear weapons, and has the capability to deploy nuclear warheads on intermediate-range ballistic missiles.

Countries could still acquire nuclear arms without tests, but they would not be confident about deploying more sophisticated warheads without testing them first.

A serious barrier to further Russian disarmament below 1500 deployed strategic warheads has just been removed with the scrapping of the Bush missile defence scheme.

The administration would cut the number of operationally deployed warheads over the next decade by two-thirds - to between 1,700 and 2,200.

An estimate by Hans Kristensen and Robert Norris estimate Russia has approximately 2,000 deployed tactical warheads.