US President Barack Obama and his Russian counterpart Dmitry Medvedev are due to sign a landmark¹ nuclear arms treaty in the Czech capital Prague.

美国总统奥巴马与俄罗斯总统梅德韦杰夫将于捷克首都布拉格签署具有里程碑意义的核武器协议。

Last-minute preparations for the signing have been under way in Prague

The deal, agreed last month, commits the former Cold War foes²（敌人，反对者） to cut their stockpiles（库存，储备） to 1,550 warheads（弹头） each, about 30% less than currently allowed.

It also limits the number of launchers each country has.

The treaty replaces the 1991 Strategic Arms Reduction Treaty (Start削减战略武器条约), which expired（期满，失效） last December.

The new limits must be implemented³（执行，实施） within seven years of the treaty coming into force.

After it was agreed, President Obama hailed the deal as the most comprehensive weapons control agreement in nearly two decades.

"With this agreement, the United States and Russia - the two largest nuclear powers in the world - also send a clear signal that we intend to lead," he said.

Missile defence

Russia's Foreign Minister Sergey Lavrov said the treaty marked a "new level of trust" between the two countries.

But he has also warned this week that Russia could opt⁴ out of（决定不参加） the treaty if it felt threatened by US missile defence plans.

It was Moscow's concerns over Washington's missile shield plans that delayed the current agreement.

US President Barack Obama scrapped⁵（废弃） a previous plan for a missile shield based in Poland and the Czech Republic that had angered Russia.

Mr Lavrov said Washington's current plans - which could include ground-based interceptor missiles in Romania - seemed acceptable to Moscow.

Correspondents say the treaty marks the first step in President Obama's ambitious（有雄心的，热望的） arms control agenda.

It was in a keynote speech in Prague last April that he set out his vision of moving towards a world free of nuclear weapons.

US nuclear proliferation（核扩散） analyst⁶ Joseph Cirincione said the treaty also signalled a major improvement in relations between the two superpowers.

"This treaty is a major re-set of US-Russian relations which were at Cold War levels just 18 months ago after the Georgian war," he said.

In a further development on Tuesday, President Obama unveiled the US Nuclear Posture⁷ Review, which narrows the circumstances in which the US would use nuclear arms.

For the first time the US is ruling out（排除） a nuclear response to attacks involving biological, chemical or conventional weapons（常规武器） .

However, countries will only be spared a US nuclear response if they comply with the Nuclear Non-Proliferation Treaty - this does not include Iran and North Korea.