The US and Russia have met in Prague to sign a new Strategic Arms Reduction Treaty (START) to cut their deployed nuclear weapons stockpiles.
The pact, known as New START, establishes comprehensive procedures for both countries to verify each others' weapons and sets new bounds on ready-to-use, long-range nuclear missiles.
President Barack Obama said New START has set significant reductions in the nuclear weapons both nations will deploy, while cutting their delivery vehicles by around half.
Obama said the treaty enabled both nations the flexibility to protect their security, and added that America remains committed to the security of its European allies.
"The nuclear weapons threaten the common security of all nations," he said.
In addition, the new treaty sets the stage for talks about further reducing both countries' strategic and tactical weapons, including non-deployed ones.
Both nations have agreed to expand their discussions about missile defence, including regular information exchanges about threat assessments and a joint assessment of emerging ballistic missiles.
A nuclear diplomacy meeting involving representatives of 47 nations will be held in Washington next week to discuss concrete steps that help secure vulnerable nuclear materials around the world within four years.