A Russian arms control official said for the first time on Friday that there was not enough time to replace the last and most important nuclear arms-limitation treaty with the United States before it expires early in 2021, raising the possibility that Washington and Moscow would then be free to expand their arsenals without limits.

The fate of the New Start treaty has been in considerable doubt since President Trump pulled the United States out of another treaty, the Intermediate Nuclear Forces agreement, this year. But New Start is considered a far more critical agreement because it limits the number of strategic weapons — the most powerful weapons both sides can launch from submarines and bombers and on intercontinental ballistic missiles.

At the height of the Cold War, the Soviet Union and the United States collectively possessed tens of thousands of nuclear weapons. After a series of arms-reduction treaties, New Start, which went into effect in 2011, limited the strategic arsenals to 1,550 each. (Smaller tactical weapons were not covered.)

New Start expires in February 2021, just weeks after the next presidential inauguration in Washington. While it can be extended for five years by mutual agreement, President Trump and his aides have signaled repeatedly that he intends to let it expire unless it can be broadened to include other nations with strategic weapons, chiefly China.