A US official focusing on arms control on Monday provided what he called new, declassified details of a Chinese underground nuclear test nearly six years ago and urged countries to press China and Russia to do more on nuclear disarmament. Christopher Yeaw, assistant secretary of state for the bureau of arms control and nonproliferation, spoke to a UN-backed body after the last nuclear arms pact between the United States and Russia expired this month. That has ended limits on the arsenals of the world's biggest nuclear powers and raised concerns about a possible new arms race.

Yeaw called for greater transparency from China and pointed to some shortcomings of the New START treaty, such as that it didn't address Russia's large arsenal of nonstrategic nuclear weapons - which counts up to 2,000 warheads. "But perhaps its greatest flaw was that New START did not account for the unprecedented, deliberate, rapid and opaque nuclear weapons buildup by China," he told the UN-backed Conference on Disarmament.