North Korea has fired a long-range rocket, which critics say is a test of banned missile technology.
The country had notified UN agencies in advance that it intended to put an earth observation satellite into orbit.
Both South Korea's military and a US defence official speaking to Reuters news agency confirmed the launch.
It appears that the rocket was fired from a missile base in the north-west of the country and passed over Japan's southern Okinawa islands.
Analysts believe Pyongyang is developing nuclear weapons capable of reaching the US mainland.
Japanese Prime Minister Shinzo Abe called the launch "absolutely unacceptable," saying it was a "clear violation" of UN Security Council resolutions.
UN Security Council resolutions ban the state from carrying out any nuclear or ballistic missile tests.
South Korean analysts had speculated that the North might do the launch ahead of 16 February, the birthday of the late North Korean dictator Kim Jong Il.
North Korea has already provoked international criticism this year with a fourth nuclear bomb test on 6 January.
The North insists its space programme is purely scientific in nature, but the US, South Korea and even ally China say the rocket launches are aimed at developing an inter-continental ballistic missile capable of striking the US.