Leader Kim Jong-un said further tests were not needed, as the North had demonstrated it had nuclear weapons.
Donald Trump described the move as "good news" for the world, and South Korea said it was meaningful progress.
The EU said it was "positive", but called for complete denuclearisation. North Korea is preparing for historic summits with South Korea and the US.
Early on Saturday Kim Jong-un said: "From 21 April, North Korea will stop nuclear tests and launches of intercontinental ballistic missiles."