The US Senate has pushed the ratification of the START treaty back to September.
The treaty, originally signed by presidents Dmitry Medvedev and Barack Obama in April, spelled out the terms under which each country's nuclear arsenals would gradually be slashed by a third.
However, it will come into force until ratified by both sides. So far, neither side has done that.
Moscow says it's ready to put the deal into effect, but wants to do so at the same time as Washington.
Chuck Hagel, influential former US Senator, attributes the delay to the natural difficulty of preparing to undertake the responsibility for such a serious agreement.
“I don’t think there is any question that the votes are there in the US Senate to ratify this treaty,” Hagel stated. “But I think it’s a wise course of action to take a little more time.”