The US and Russia hold some 95 per cent of the world's nuclear arms

Rachel Courtland, reporter

After almost a year of negotiation, the US and Russia have finally settled on a plan to further reduce their vast nuclear arsenals. If the new agreement is approved, the number of deployed warheads by will be cut by 30 per cent over previous targets. But an ongoing dispute over a European missile defence shield has the potential to scupper the plans.

The US and Russia hold some 95 per cent of the world's nuclear arms, including an estimated 4700 deployed nuclear warheads. The new agreement – a follow-up to the 1991 Strategic Arms Reduction Treaty (START) – would cut the number to 1550 warheads per country over the course of seven years.

The significance of the new agreement is largely symbolic. A fraction of those warheads could still assure mutual destruction, Vanity Fair notes.

But US president Barack Obama and Russian president Dmitry Medvedev hope the new START will show other nations that the world's nuclear superpowers are serious about disarmament. A review of the Nuclear Non-Proliferation Treaty, which has met with faltering support, will be hosted by the US in May.

The US Senate and Russia's Duma must ratify the new START treaty before it can take effect. But this may prove difficult, as some American legislators have expressed concern over Russia's attitude toward the treaty.

While the two countries have agreed on a new plan for offensive weapons, they differ on the subject of missile defence. The US hopes to install a shield of interceptor missiles in Europe to protect against potential attacks from countries like Iran.

Russia suspects the shield is really there to block its own missiles, and in a separate, nonbinding statement has declared it reserves the right to withdraw from START if US missile defence plans seem to threaten its national security, the New York Times reports.