The U.S President Barack Obama and his Russian counterpart Dmitry Medvedev are due to sign a landmark nuclear arms treaty in the Czech capital Prague. The deal, agreed last month, commits the former Cold War foes to cut their stockpiles to 1550 warheads each, about 30 per cent less than currently allowed. It also limits the number of launchers each country has. The treaty replaces the 1991 Strategic Arms Reduction Treaty (START) which expired last December. The new limits must be implemented 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. Russian Foreign Minister Sergey Lavrov said the treaty marked a new level of trust between the two countries. But he has also warned that Russia could opt out of 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. U.S President Barack Obama scrapped a previous plan for a missile shield based in Poland and the Czech Republic that had angered Russia.