Russian Prime Minister Vladimir Putin said today that U.S. plans for a missile defence system were the main obstacle to reaching a new deal on reducing Cold War arsenals of nuclear weapons, according to Reuters. The two largest nuclear powers say they are close to agreeing on a successor to the 1991 Strategic Arms Reduction Treaty (START I), although U.S. President Barack Obama and Russian President Dmitry Medvedev have yet to clinch a deal. Asked by a reporter what the biggest problem was in the talks, Putin said: "What is the problem? The problem is that our American partners are building an anti-missile shield and we are not building one." Speaking to reporters in the Far Eastern Russian city of Vladivostok, Putin said the U.S. plans would fundamentally disrupt the Cold War balance of power and Russia would thus be forced to develop new offensive weapons. The comments, from Russia"s most powerful politician, showed the seriousness of the problems hampering talks on a replacement for START I and illustrated the deep unease still felt in Moscow over Washington"s missile defence plans. In Washington, a senior U.S. official told Reuters: "We have made substantial progress in the negotiations and remain confident that when talks resume in January that we"ll be able to finalize an agreement." In September, Obama said the United States would scrap parts of George W. Bush"s missile defence plans, a step seen as an attempt to allay Kremlin fears that the system was a direct threat to Russia. Cutting the thousands of nuclear weapons accumulated during the Cold War is the centrepiece of Obama"s efforts to "reset" relations with Russia.