Announcing the move, Secretary of State Antony Blinken said it made the world safer.
President Joe Biden and his Russian counterpart Vladimir Putin reportedly discussed it by phone a week ago, and Mr Putin signed it into law on Friday.
The treaty had been set to lapse on 5 February, as the Trump administration had refused to approve the extension.
It had sought tougher verification procedures and some inclusion of tactical weapons, as well as an expansion of the deal to cover China.
New Start is the last remaining nuclear arms deal between Russia and the US, and its demise would have ended all limits on deployments of strategic nuclear warheads and delivery systems."This welcome step is the start of our efforts to pursue effective arms control that lowers the risks of war and helps prevent arms races."
A Russian foreign ministry statement said the agreement entered into force on Wednesday after diplomatic notes were exchanged with the US Embassy in Moscow, adding that the treaty remained in effect "exactly as it had been signed, without any amendments or additions".Originally signed in 2010, the treaty limits each side to 1,550 long-range nuclear warheads, a lower number than under the previous Start deal.
Each country is allowed, in total, no more than 700 deployed intercontinental ballistic missiles, submarine-launched ballistic missiles, and heavy bombers equipped for nuclear arms.
Another 100 are allowed if they are not operationally deployed - for example, missiles removed from a sub undergoing a long-term overhaul.
Again, this is a significant reduction from the original treaty.
source:bbc.com
